Acknowledgments
Writing a book is a daunting task, and it is never the work of a single person. I would like to take the time to thank several people who were indispensable in making this project happen: Tomer and Yehuda Shiran, who wrote the first edition of this book. They gave me a fabulous foundation to work with for this second edition. Jim Hill and Wes Beckwith, the wonderful people at Wordware Publishing who labored through this with me. Derick Richards, who did a technical review of this book. Last but not at all least, my wife, Misty, who did not complain at all when I was spending hours in my den typing away.

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Introduction
Prerequisites
JavaScript is a scripting language designed to be used within HTML documents. For this reason a basic working knowledge of HTML is required before you can learn or use JavaScript. For those readers either lacking this basic knowledge or requiring a refresher course, Chapter 2 is a basic HTML primer that will teach you all of the skills you will need.

A Programming Refresher
Chapter 3 is a brief crash course in basic JavaScript to help readers with no prior knowledge of JavaScript as well as those who require a brief refresher. This chapter covers all the fundamental elements of JavaScript and will prepare the reader to continue on with the material in this book.

How to Read This Book
As the title suggests, this book is geared towards advanced JavaScript programming and has the experienced JavaScript programmer in mind. However, the first few chapters do provide basic primers on both HTML and JavaScript. Those with limited JavaScript experience should carefully study the first few chapters, while more experienced programmers may want to skim these chapters or skip them entirely.

The Structure of This Book
It’s important to understand how the book is structured so you can plan your learning experience in the best possible way. The table of contents lists the chapters by name. If you are an experienced JavaScript programmer just looking for a reference on a particular subject, feel free to skip directly to the relevant chapter. JavaScript is a viable scripting language for both Netscape and Internet Explorer, so I will use examples from both browsers and will point out any differences. If you have the latest version of either Netscape or Internet Explorer, then virtually all the

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examples should work fine for you. Older versions of both browsers, particularly 3.0 and earlier, lacked support for some JavaScript features.

Programming Style
As both an aid to the reader and to illustrate good programming practices, I use a uniform style throughout this entire text and in all the sample code you will find on the companion CD. For example, all variables are in Hungarian notation, meaning the variable is preceded by one to three letters designating the type of variable it is. For example, an integer being used as a loop counter might be named intcounter, and a float that holds an account balance might be named fltbalance. I also have a tendency to comment very heavily, some would say too much. This comes from teaching and creating code samples for students. I hope you will find this helpful:
for (var intcounter = 0; intcounter < 10; ++intcounter) { [JavaScript statements] }// end of for loop

A lot of my style preferences are just that, preferences. It is hard to call one style right and another wrong. However, I strongly suggest that when you are writing code you consider the fact that other programmers will probably need to read your code at some point. They will have absolutely no idea what you were thinking when you wrote your code. I try to write code in such a way that even a novice programmer with no prior knowledge of the project at hand could easily deduce the intended purpose of the code.

Reading Examples
It seems obvious but read the examples thoroughly. Make sure you have a firm understanding of how and why they work. It is also helpful to actually run them. If you don’t wish to run them, at least read them thoroughly and make certain that you fully understand every line of code and what it does.

Encouragement for Nonprogrammers
Learning a scripting language is a lot easier than learning a full programming language such as Java or C++. Fortunately, JavaScript is enough like Java that you will find it relatively easy to leverage your JavaScript skills into Java programming as well. Just remember that computers do exactly what we tell them to do, which may not always be exactly what we intended them to do. Most programming errors (called “bugs”) stem from simple mistakes. I have an axiom that I believe is true: “Any error that

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takes more than 15 minutes to find will turn out to be something simple.” In my experience, complex or fundamental errors are easy to find, though they may be quite hard to fix. Errors that take a long time to find are almost invariably simple mistakes.

Warning for Programmers
If your programming experience has been in strictly procedural languages such as C and Pascal, hold on tight. JavaScript, like Java, is object oriented. This will be a whole new approach to programming for you. But not to worry—this book will walk you through it step by step. However, if you are an experienced object-oriented programmer, especially a C++ or Java programmer, then this book should be a pretty easy read for you.

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Chapter 1

Introduction to JavaScript
The World Wide Web
It may be hard for some readers to recall a time when the web was not a part of our day-to-day lives. However, it was not until the early ’90s that the web became a serious medium for communication. Prior to that, the major uses of the Internet were simply e-mail and file transfer. This is not to say that web pages did not exist before then; it’s simply that they were not a widespread communication tool. By the early 21st century, however, there were millions of web sites from major corporations and government agencies to 10-year-olds putting up web sites extolling the virtues of their favorite cartoon. It is hard to imagine conducting business today without some web presence, whether it’s a small web site that gives the essentials of your business or a full blown e-commerce site. This spread of the web has also lead to greater expectations from web pages. Originally, simply putting up some well-organized text was enough to give you a successful web site. But soon, web surfers became accustomed to graphics, animations, sounds, and more. All of this could be accomplished with successive versions of HTML. Modern web users, however, expect a higher level of functionality from web sites.

Competition on the Web
Site popularity is usually measured by the number of visits, or “hits,” a web site receives. Major sites such as www.yahoo.com can receive tens of thousands of hits per day. Less popular sites may only receive a few hundred hits a day, while personal web pages may only receive a few hits per week or month. The competition to get visitors to sites is becoming quite fierce. In fact, advertising dollars are now pouring in to sites that can attract a significant number of visitors. The most common way to get visitors to a site is to add content, both informational/functional and purely decorative. This is an area for which JavaScript is ideally suited. The World Wide Web (WWW) is constantly growing as new web servers and pages are connected. It is estimated that over 15,000 domain names (such as netscent.com) are

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registered daily. Each domain name is eventually associated with a web server or a virtual server. Your web site is truly useless if people do not visit it. There are several ways to attract web surfers to a site: n Advertising. You can advertise your web site on pages that belong to other companies or individuals by placing some text or images on their page. The user loads the page and sees a banner at the top of the page; when he or she clicks the banner, the advertiser’s site is loaded. Many large companies also tend to advertise via traditional methods such as newspapers, magazines, television, and radio. Although this method is somewhat less efficient, it works surprisingly well for well-established companies. n Submission. Once you have a web site, people should know that it exists. Most people and small businesses do not have enough resources to advertise via banners that can cost up to tens of thousands of dollars a month. The most common free promotion method is to submit the URL and description of a web page to search engines and directories. Those services provide an easy way of retrieving desired web pages. For example, a search engine returns URLs of pages that are related to a specific topic that the user searched for. AltaVista, Excite, and Lycos are just some examples of such search engines; there are hundreds available on the web. Directories such as Yahoo! gather web pages by subject so the user can find pages related to a given subject. n Attractive site. The most important way to attract users to your site is by creating an engaging site. Most web surfers tend to stick to sites that they have visited before and where they have found interesting, new material every visit. The most “effective” way to deter people from revisiting your site is by allowing it to become stale.

Extending HTML
HTML is a somewhat limited document-formatting language. It is based on tags, which instruct the browser how to display a chunk of text or an image. As such, the HTML is limited to a static, one-way interaction with the user. The web page can be as sophisticated and attractive to the surfer’s eyes as it can possibly be, but the user cannot interact with the page. Interaction cannot be static—it requires constructs such as if statements and for loops, which are not part of the HTML syntax. These missing constructs are found in JavaScript. This object-oriented language provides web page authors with the power to reach a very high level of interaction between the user and the document. The power of the language is best witnessed from the pace of its evolution. Browser providers are coming out with new extensions and features every few months, leapfrogging each other in their constant quest for market dominance. The leading browser provider companies are thus setting the standards in web page authoring features, with JavaScript being of special importance.

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CGI Programs
You are probably familiar with popular search engines such as Yahoo!, AltaVista, and Excite. Have you asked yourself how these engines work? They are based on CGI (Common Gateway Interface) scripts. CGI extends the interaction between the user and the document to a higher level where the document communicates with the web server on which the web pages reside. When you click the Submit button on a form, your browser sends your entries from the HTML form to the server, where the CGI script processes the data and sends it to another program running on the server. The results are then transmitted to the user’s browser. CGI scripts can be written in any language, Perl and C being the most popular ones. Objectively, Perl and C are more complicated to program than JavaScript, and therefore, CGI scripting is usually mastered only by experienced programmers. You are now in a position to understand why search engines need such powerful machines to run their devices. A typical search engine receives several million requests per day. For example, suppose a provider needs to answer one million search requests per day. A simple calculation shows that that server must process approximately ten one-second-long requests per second. Each of these ten requests is a CGI script executing on the server.

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Helpers, Plug-ins, Applets, and CGI
Web page authoring consists of several layers. The more powerful the layer, the more complex it is to program. The fundamental layer is HTML. As we explained earlier, JavaScript extends HTML to a dynamic user/page interaction. Helpers, plug-ins, and applets extend JavaScript’s capabilities even more.

Helpers
Browsers are very powerful in converting HTML tags into attractive pages, as well as interpreting JavaScript scripts to animated commercials, but they have some deficiencies in other areas such as understanding audio that comes in from a web site. Your browser, though, is smart enough to use helper applications to do such jobs instead. The conversion of digitized sound to audio you can hear through your speakers is done by a program that resides on your hard disk and is launched by the browser.

Plug-ins
Another way to extend browser capabilities is by plugging an application into it. At this stage we will not discuss plug-ins, because a full chapter is dedicated to these mechanisms and their powerful connections with JavaScript.

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Java Applets
OK, so you have tried HTML, JavaScript, helpers, and plug-ins (in this order) and you are still not satisfied with your web page. Let’s say you want to place a rotating globe on your page. Sun Microsystems answered this need by developing a language for the web called Java. Talented programmers can use Java to build small applications (applets) that are downloaded to the browser upon hitting the appropriate page and then automatically discarded from memory when a new page is loaded. The applet concept is similar to the image file concept. As the .gif file is loaded upon hitting the page, which calls the appropriate image, so is the applet loaded upon the browser’s request. The applet, though, is more than a static combination of pixels; it is an independent program that executes when the page loads and is automatically terminated when the page unloads. The applet is a self-sustained, independent module, without any possibilities for user interaction during its execution. The classic example of an applet is a scrolling LED banner, similar to the scrolling sign at Times Square. There is no better way to inform your visitors about changes you have made to your page. You can change your banner whenever you want to bring new items to the user’s attention. This touch of animation can bring your page to life and create the perception of a dynamic, up-to-date site. For a browser company to support Java applets, it needs to license the technology from Sun and build the proper hooks in the browser. The first one to do it was Sun itself, with its HotJava browser. Netscape incorporated it later, as did Microsoft. Java applets are assets that people are accumulating, and every browser must support them to keep up with the nonstop competition. You have probably asked yourself about the origin of Java. The Java language is derived from C and C++ and is targeted for more experienced programmers. Therefore, many web page authors and casual programmers cannot adopt the language as easily as they have adopted HyperCard, ToolBook, or Visual Basic. JavaScript was created to answer this exact need.

CGI (Common Gateway Interface)
There are some tasks that cannot be accomplished by the browser on the client side. Building a common database for an engineering department must rely on a server accepting data and requests from all users and storing it for future access and processing. Such an application can be written in C, C++, Perl, or any other language. The communication protocol between the server application and the client browser is called CGI. You should buy a book on CGI if you intend to learn it, because it is far beyond the scope of this book and is very difficult to learn without detailed documentation.

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What is JavaScript?
JavaScript is an easy-to-use object scripting language designed for creating live online applications that link together objects and resources on both clients and servers. While Java is used by programmers to create new objects and applets, JavaScript is designed for use by HTML page authors and enterprise application developers to dynamically script the behavior of objects running on either the client or the server. JavaScript’s design and concepts represent the next generation of software for the Internet. JavaScript is: n Designed for creating network-centric applications n Complementary to and integrated with Java n Complementary to and integrated with HTML n Open and cross-platform With JavaScript, an HTML page might contain a form that processes data on the client side. A server-side JavaScript might pull data out of a relational database and format it in HTML on the fly. A page might contain JavaScript scripts that run on both the client and the server.

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Who Can Benefit from JavaScript?
Generally speaking, web surfers benefit most from JavaScript, because they have the opportunity to enjoy the language’s capabilities in the form of games, animation, interaction, and so forth. Netscape Communications benefited from JavaScript early on, because it gave them a competitive advantage over other web browsers. Now, however, both Microsoft Internet Explorer and Netscape Navigator fully support JavaScript. Many other companies believe that they will benefit from JavaScript and its descendants. The words of Rose Ann Giordano, vice president of the Internet Business Group at Digital Equipment Corp., demonstrate it: “Tools like JavaScript will unleash a new wave of creativity and transform the Internet in ways no one can predict. JavaScript and other developments will demand increased system performance, ideally met by Digital’s Alpha systems architecture.” Jan Silverman at Hewlett-Packard also mentioned how JavaScript matches his company’s plans in a press release: “JavaScript is an exciting technology because it represents the next generation of software designed specifically for the Internet. Hewlett-Packard is committed to open standards and is a supporter of JavaScript because it complements Hewlett-Packard’s open systems architecture.”

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The History of JavaScript
JavaScript was originated by Netscape as LiveScript, developed in parallel with LiveWire server software. LiveScript was developed for several applications—we’ll discuss two of them here. The first one is to enhance web pages in a way that HTML cannot. The classic example is verifying a user’s form entries. Instead of sending the data as is to the server and validating the data types there, the client handles all the validation and only then sends the data to the server for further processing. Another application for LiveScript (JavaScript) is as the communication glue between HTML documents and Java applets. A scrolling banner, for example, can use information from the user’s customized settings, sent to the applet by LiveScript. This data exchange is transparent to the server and, since there is no server-client communication, there is no response time penalty. In December 1995, Sun Microsystems took over LiveScript development and changed its name to JavaScript. On the one hand, JavaScript is related to Java in its C++ object flavor. On the other hand, JavaScript’s vernacular is much reduced compared to Java, in order to make it suitable for less experienced programmers and scripters. JavaScript’s first and foremost advantage is in its ease of learning. Additionally, its features and capabilities are important for web page interactivity as well as for customizing solutions around prewritten applets. In 1996 Microsoft introduced its first JavaScript-enabled browser, Internet Explorer 3.0. Netscape followed up a week later with another JavaScript-enabled browser, Netscape Navigator. Internet Explorer was far from reaching Navigator 3.0’s level in terms of JavaScript and lacked many important features and capabilities. In early 1997 the beta version of Netscape Navigator version 4.0 (Communicator) was released. Microsoft Internet Explorer was still far from reaching Navigator 3.0’s level. At this point, Internet Explorer 5.5 and Navigator 6.0 both fully support JavaScript.

JavaScript in a Browser
Client-side JavaScript
Client-side JavaScript is the most common form of the language. The script should be included in or referenced by an HTML document for the code to be interpreted by the browser. Suppose you load a web page with a JavaScript-generated banner. The browser begins interpreting the HTML code and then encounters a JavaScript script. If the script has no syntax errors, it is executed by the browser’s built-in interpreter. The JavaScript client-side mechanism features many advantages over traditional CGI server-side scripts. For example, you might use JavaScript to check if the user has entered a valid e-mail address in a form field. The JavaScript function is executed when the user submits the form, and only if the entry is a valid e-mail address (includes an @) is the form transmitted to the server for further processing, database storage, and so on. In this case, the user does not need to wait until the form is

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submitted over the network to the server and back to the client, only to be informed that a single character is missing.

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Browser Objects
JavaScript is an object-based programming language. Its built-in object model is mostly based on the HTML content of the given web page. The tight interaction between JavaScript and other browser objects (such as forms, browser windows, frames, and images) provides full control over various page elements and enables the programmer to create a link between “external” objects and “internal” ones. A classic example for such linking is a JavaScript script that invokes a Java applet from an HTML form. JavaScript exposes objects and properties related to the browser’s window, history list, status bar, frames, forms, links, and so forth. Furthermore, JavaScript can be used to trap user-initiated events such as button clicks, link navigation, and other actions that the user explicitly or implicitly initiates. You can create a distinct script for each event, enabling a smooth, logical interaction with the user.

What Can JavaScript Do?
In this section we take a look at a few interesting effects and programs created entirely with JavaScript. At this point we will only focus on client-side JavaScript that is embedded in or referenced by an HTML document.

Games
You can create many interesting games with JavaScript. During our JavaScript programming experience we have written tic-tac-toe games, a mastermind game, a Tetris game, and many others. There are basically two types of games you can create via JavaScript: n Static games n Dynamic games Static games are usually logical ones, and thus do not require much action or immediate responses. For example, you can play tic-tac-toe without any live action, because even reloading the entire page for each move does not interfere with the game itself (although it might be very boring). Static games do not require image replacements or animation. Dynamic games, on the other hand, require real-time action and animation. For example, a Tetris game requires that the blocks will fall down the screen smoothly without having to wait each time for the screen to be refreshed. Dynamic games are often resource intensive and therefore must be programmed with efficiency in mind. Small, simple games are usually embedded in existing web pages, whereas large, complex games are most often placed on a separate page.

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Banners
JavaScript is a full scripting language, so you can create an infinite number of distinct scripts. Nevertheless, some implementations became more popular than others due to the fact that they are easy to use and create interesting effects. Banners are a classic example of such scripts. A banner is a script that displays text in the browser’s status bar or in a form’s text field. The most common browsers are those that scroll and those that type in messages sequentially.

Form Validation
JavaScript is tightly connected to browser objects, including forms and their elements; therefore, it provides a great amount of control over forms. A classic form-related script is one that validates a form’s fields before it is submitted and cancels the submission to the server if an error is found in one of the fields. The advantage of using JavaScript in this particular example is that client-side validation is much faster than validation via transmission to the server and back to the client. For example, suppose you have a form that accepts data from the user, including his or her e-mail address. Let’s assume that the e-mail address is essential, and that a string containing an “@” character is assumed to be a valid e-mail address. There are two options: n You can choose not to validate the entries. In that case, when the user clicks the Submit button, the form is submitted to the server where a CGI script checks if the e-mail address field is valid and then processes the form. If the e-mail address does not contain an “@” character, the submission is denied and the appropriate message is returned to the user. n You can choose to use client-side validation via JavaScript. In that case the validating script is loaded as plain text (JavaScript code embedded in the HTML code). When the user clicks the Submit button, a JavaScript function is triggered to validate the form. If the e-mail address seems valid, the form is submitted to the server—otherwise an appropriate message is immediately displayed and the submission is canceled. Note that the user does not have to wait for the form to be submitted to the server and back to the client, only to receive a message informing him or her that the e-mail address field contains an invalid entry.

Miscellaneous
I have covered three important JavaScript implementations. However, like all other languages, it is obviously impossible to cover all or most of the possible scripts. You can use JavaScript to create almost anything you desire. For example, you can create an LED sign, which once could be created only with Java. Bear in mind that client-side JavaScript is executed on the user’s machine. That is, you do not have any direct access to the server’s properties with client-side JavaScript. Although that may seem to be a disadvantage, it is sometimes very convenient to have access to client-side properties rather than server-side ones. For example, you can create a digital clock that shows the current time in the client’s time

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zone, because JavaScript extracts the time from the client’s operating system, be it Windows 95/98/2000, Macintosh, Unix, or any other OS. The full control over browser objects is what makes client-side JavaScript so powerful. You can pop up windows and use them as remote controls linked to the original window. You can use one link to perform several operations or load various documents in separate frames simultaneously. You can also use JavaScript to create an online calculator or to draw graphs and curves. To attract visitors, you can enhance a web page with JavaScript in many different ways. We have introduced a few client-side JavaScript scripts in this section. Although clientside JavaScript is by far the most important and useful, there are many other implementations of the language. For example, you can use JavaScript for server-side applications using the LiveWire compiler. Server-side JavaScript is actually an alternative to traditional CGI programming via Perl and C++. JavaScript is a very convenient language; thus, it is used alongside many other languages (such as VRML) and for various purposes. Another example of using JavaScript is for automatic proxy configuration. As you might already know, Netscape Navigator enables you to connect through a proxy server. You can configure the proxies manually by entering the correct values or entering the URL of an automatic proxy configuration script, which is actually a JavaScript code. The JavaScript code is located in a text file on the server but is not compiled like server-side JavaScript. You should know by now that there is plenty to do with JavaScript, and JavaScript is now one of the most important languages for web development. We will cover the most important implementations of the language in this book.

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JavaScript and Java
JavaScript Resembles Java
JavaScript supports most of Java’s expression syntax and basic control flow constructs. Take a look at the following JavaScript code segment:
for (var i = 0; i < 10; ++i) { /* statements come here */ }

Now take a look at the Java equivalent:
for (int i = 0; i < 10; ++i) { /* statements come here */ }

Notice the similarity. The only difference is the variable declaration (JavaScript is loosely typed).

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JavaScript and Java are both based on objects, but their implementations differ. In both languages, many built-in functions are implemented as properties and methods of various objects.

JavaScript Differs from Java
JavaScript resembles Perl in that it is interpreted, not compiled. Java is referred to as a compiled language. Unlike most other programming languages, though, Java is not compiled to a native machine code, but rather to a Java byte code. Java byte code is an architecture-neutral byte-code compiled language. That is, an applet is compiled to Java byte code and then run by a machine-dependent runtime interpreter. Therefore, Java is much slower than general programming languages such as C++. Since Java is compiled, the common user cannot see the actual code behind the program. Nevertheless, when a user comes across a JavaScript script, he or she can generally see and even copy (legally or illegally) the code simply by using the browser to view the HTML source that contains the script (unless it is an external script). A compiled language has many other advantages. For example, a compiled program is much more efficient than one that is always interpreted directly from a text file, such as Perl and JavaScript. On the other hand, there are more than enough reasons to prefer an interpreted language over a compiled one. It is much easier and more convenient to debug and modify a program by simply modifying its text file rather than having to recompile it. Furthermore, most scripts and applets implemented in Java or JavaScript for web usage do not require efficiency and do not demand resources. Therefore, an interpreted language is somewhat more convenient. Both Java and JavaScript are based on objects. However, their implementations of objects are different. JavaScript is an object-based language. It supports built-in, extensible objects, but no classes or inheritance. Its object hierarchy is not an inheritance hierarchy as in Java. JavaScript features two types of objects: n Static objects—objects that combine general functions (methods) and data constructs (properties). The values of such objects’ properties are usually read-only. A static object is a single object, and thus does not enable you to create instances of it. For example, the Math object in JavaScript is a static one, because you cannot create instances according to its template. Its methods are common mathematical functions, whereas its properties are mostly mathematical constants. n Dynamic objects—objects by which instances are created. A dynamic object resembles a template. You do not touch or use the object directly. In order to take advantage of such an object, you must create an instance of it. For example, the Date object is a dynamic object in JavaScript. An instance of that object is associated with a given date. You can create as many instances of a dynamic object as needed. Java is fully extensible. A programmer can create numerous classes that group objects together. A class is a term used in object-oriented programming vernacular to refer to a set of related objects that share common characteristics. Java programmers create their own extensions to the base set of tools or classes. JavaScript’s object model is

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somewhat simpler than Java’s equivalent, so JavaScript’s object implementation is much easier to accommodate than Java’s model. Another difference between Java and JavaScript is their time of binding. JavaScript features dynamic binding, so all object references are checked at run time. Java, on the other hand, is based on static binding, meaning that all object references must exist and be valid at compile time. However, an object-oriented language may require dynamic method bindings because polymorphism allows multiple definitions of methods sharing a common name, and calling such polymorphic methods often cannot be resolved until run time. The most obvious reason for this difference is that JavaScript is not compiled, so checking object references at compile time has no meaning. If you are not familiar with Java, you should pay no attention to these confusing terms—simply bear in mind that the object implementation in JavaScript varies widely from that in Java. When you write a script, you likely need to use variables. In order to use a variable, you must declare it. Another difference between Java and JavaScript is that Java is strongly typed, as opposed to JavaScript, which is loosely typed. That is, when you declare a variable in Java, you must specify its data type. For example, when you create a variable to contain integer values you must use the int keyword. In JavaScript, all variables are declared in the same way. Furthermore, a variable of one data type can contain a value of a different data type elsewhere in the script. This behavior resembles other popular languages such as Perl. This topic is discussed later in the book. I have intentionally left the most critical difference to the end. You may have noticed that all differences discussed thus far are related to the language itself. A very important difference between Java and JavaScript is that JavaScript is integrated with, and embedded in, HTML. Although Netscape Navigator 3.0 introduced an alternative in the form of external files, that is still the general situation. Java, conversely, is neither integrated with nor embedded in HTML. Applets (small Java programs) are always distinct from the HTML code and are only invoked by it. A space is reserved on the web page for later execution if needed. If you have some experience creating HTML documents, JavaScript’s implementations will be a piece of cake for you. When you create the HTML document for a web page, you simply embed the JavaScript script according to its syntax and semantics. Another significant difference between Java and JavaScript is the area in which they are executed. A Java applet resembles an image in that you give it a specified area on the page, and the applet executes independently in that area. Within the defined area, the applet can do virtually anything, including animation, games, and so forth. An applet does not have any effect outside its area. JavaScript, on the contrary, gives you access to the entire web page. You can affect various elements in the page, provided that JavaScript is given permission to access them. For example, you can reference any form, link, image, and browser window via JavaScript. Netscape Navigator 3.0 introduced a new feature, LiveConnect, which enables Java to interact with JavaScript and vice versa. This tool makes it possible for a Java applet to interact with HTML elements through JavaScript, which is the connector in this case.

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Java is not just a language for creating applets that run on web pages; it is also a general-purpose language that can create stand-alone applications. That is, you can create any application with Java, without any connection to the Internet or the World Wide web. The classic example for a stand-alone application written entirely in Java is HotJava, the web browser created by Sun Microsystems. On the other hand, JavaScript is not a stand-alone programming language. Like most scripting languages, JavaScript is interpreted and thus requires a specific application to run it. JavaScript currently works with Netscape Navigator, Microsoft Internet Explorer, the LiveWire server environment, and plug-ins. JavaScript applications will probably never function independently. Java requires an appropriate multitasking operating system. Therefore, anyone running Unix platform, OS/2, Windows 95/98/2000, Windows NT, or Macintosh should not have a problem viewing Java on the web. Nonetheless, Windows 3.1 or Windows 3.11 users cannot benefit from Java because that operating system does not support multitasking. Also, some users running multitasking operating systems cannot run Java applets from a web page. This happens when a user is connected via a proxy server that disables Java due to possible security flaws. As opposed to Java, JavaScript’s audience is not limited due to any operating system. Windows 3.1x users can enjoy JavaScript-enhanced pages exactly like other users. That is obviously a major benefit of JavaScript over Java.

JavaScript and Java are Complementary Languages
I have spoken with many people who believe that JavaScript is a scaled-down version of Java. As you should already know, that is not true. JavaScript cannot perform many tasks that Java can, whereas Java cannot do most things that JavaScript is capable of doing. As of Netscape Navigator 3.0x, JavaScript and Java can interact together using LiveConnect.

JavaScript’s Current Status
JavaScript is currently under development and will probably stay that way, as the language specification is not yet complete. Each browser release introduces new features to the language. Therefore, you must take special precautions when creating scripts for the general public. First of all, you must follow the rules that enable you to hide the code from browsers that do not support JavaScript. Secondly, if the script works only on some JavaScript-enabled browsers, you must take that into consideration. You can use all versions of JavaScript to find out which browser the user is running, enabling you to provide a script compatible with the browser being used. If you implement this technique, you can avoid all errors by disabling the script if it is not compatible with the user’s browser. In any case, you must assure that the user does not receive any annoying JavaScript errors that are an indication of a poorly designed web site. The fact that JavaScript is under development has immense influence because the language is an interpreted language, not a compiled one. When using a compiled language, you must use an up-to-date compiler so it can compile your code. However, once the

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JavaScript is supported by many companies. At the same time Netscape Communications and Sun Microsystems announced JavaScript, more than 28 companies also announced that they would be endorsing JavaScript as the open scripting standard for the Internet, and many companies indicated that they were considering licensing the technology to include it in their own products.

The Future of JavaScript
JavaScript is expected to advance in two areas. The first one is in the direction of “serverless” CGI scripting. Small databases of information will be embedded in an HTML document and processed by JavaScript scripts. Cutting the server-client communication will sharply reduce the response time, which is still the number one problem in surfing the Internet. The second vector is pointing to Java applets. As applets become more and more powerful, more exciting opportunities will be available to use JavaScript to assemble existing building blocks. As ActiveX and VRML become more powerful, JavaScript will work in concert with them, resulting in very powerful applications. JavaScript, a cross-platform language, is currently in use alongside many other languages, including Java and VRML. Furthermore, it has many other uses, such as automatic proxy setting scripts. In the future, other programs and languages will feature interaction with JavaScript. For example, Netscape is currently using JavaScript with LivePayment, a program that enables a web site developer to receive immediate payments over the web. A web page designer cannot afford to ignore JavaScript because his competitors will not, beating him in web design competition. Nevertheless, JavaScript has its limits and cannot perform every task. A few classic JavaScript-based solutions are as follows: n User interaction with form elements (input fields, text areas, buttons, radio buttons, check boxes, and selection lists) n Distributing small collections of database-like information with friendly interfaces to that data n Processing the data on the client side before submission to the server n Animation and live image swapping This list is obviously an abbreviated one and does not include all possible uses of JavaScript.

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program is compiled, it will run on any machine with the designated operating system. Therefore, your script must be suitable for all browsers, or at least the most popular ones. This problem is specific for JavaScript, not for all interpreted languages. For example, when you create a CGI script via Perl, you must only make sure that the Perl interpreter installed on the server meets the script’s needs. If so, it will work for any browser that supports CGI (more than 99.99% of all browsers).

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Chapter 2

HTML Primer
This chapter is designed to give you the basics of HTML. If you have no prior knowledge of HTML, then it is critical that you thoroughly study this chapter before proceeding. However if you are an experienced HTML programmer, you may wish to skim this chapter or skip it entirely. JavaScript is a scripting language that is embedded into HTML documents in order to add significant functionality to those web pages. For this reason, a working knowledge of HTML is integral to understanding and using JavaScript. HTML, or Hypertext Markup Language, is a relatively simple scripting language that web browsers can use to display web pages. The beautiful part of HTML is that you don’t need any special software to do it; HTML can be written in any text editor, including the Windows Notepad. Just remember to save the file as an .htm or .html file. HTML has had a long history and has gone through a number of revisions. Each successive revision adds more functionality to HTML and with the current version (Version 4.0), HTML is a very powerful language that can take some time to learn. Fortunately, most work on web pages can be done with just the essentials of HTML and that is what this chapter will teach you. Now let’s start with the basics of HTML.

HTML Tags
The first question, is how do we get the web browser to know that our document has HTML codes for it to read? Simply put, there is more to creating a web page than simply taking a document and changing its extension to .htm. At the beginning of your document you place the <HTML> command and at the end you put </HTML>; the web browser will know that the codes in between are supposed to be HTML.
<HTML> put code here </HTML>

Now that is pretty simple you must admit. But this web page won’t do much of anything at all. So let’s do the obligatory “Hello World” sample that every programming book starts off with. It will show you how to input text and some basic HTML.

Believe it or not, this little snippet shows you most of what you need to know about HTML. To begin with, note that everything is contained between the <HTML> and </HTML> tags. These two commands define the beginning and the end of the HTML document. The web browser will ignore any items outside these commands. Next we have a section that is contained between the <HEAD> and </HEAD> commands. This is the header portion of your HTML document. The <TITLE> and </TITLE> commands contain the title that will actually appear in the title bar of your browser. Then we have the <BODY> and </BODY> commands. As you might have guessed, this is the body of your HTML document. This is where all of your web page’s code (including scripting code like JavaScript) is going to go. Inside the body section is some text and some additional commands that will define how the text will appear in the browser. The <P> and </P> commands define the beginning and the end of a paragraph. The <B> and </B> commands tell the browser to make whatever text is between them bold. The <FONT SIZE="+2"> tells the browser how big the text should be (there are a variety of methods for doing this, as we shall see). The </FONT> command ends the font section of the HTML code. By now I trust you have noticed a pattern. All the above commands have an opening command and a closing command. This is true for all but very few HTML commands. Just remember this rule: You close the commands in opposite order of how you opened them. Notice in the above sample code I opened the commands before the text like this: <P><CENTER><B><FONT SIZE="+2">, and then closed them like this: </FONT></B></CENTER></P>. This is important to remember. You can think of it as “backing out” of your commands. Now this gives you a very simple web page that displays one phrase in bold. Admittedly, this is not very impressive, but if you understand the concepts involved with using the HTML commands, then you understand HTML. Now let us move on to some more HTML. Usually web pages contain more than simply a title and some text. Other items you might put in a web page include images and links to other web pages. Placing an image on an HTML document is rather simple:
<IMG SRC="imagepath\imagename" WIDTH=52 HEIGHT=88 ALIGN=bottom>

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You simply provide the path to the image and the name of the image, including its file extension (such as .gif, .bmp, .jpg, etc.). The other properties in this command allow you to alter the placement and size of the image. You can alter its width and height as well as its alignment.

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Placing a hyperlink to another web site or to an e-mail address is just as simple:
<A HREF="http://www.wordware.com">

This link will connect to the URL (Uniform Resource Locator) contained within the quotation marks. In order to use this methodology to create an e-mail link, simply use this:
<A HREF="mailto:somebody@somemail.com">

You simply have to change the “http://” portion to “mailto:”. Notice that all three of the preceding methods have one thing in common. They do not close the command in the typical manner that other HTML commands are closed. Now let’s examine the source code for a simple but complete HTML document:
<HTML> <HEAD> <TITLE>Test HTML Page</TITLE> </HEAD> <BODY BGCOLOR="white"> <B><FONT SIZE="+2"></FONT></B> <P><CENTER><B><FONT SIZE="+2">My First Web Page </FONT></B> </CENTER> <P>I am learning HTML. I <B><I>LOVE</I></B> HTML! </P> <P><CENTER><IMG SRC="CompuBook.gif" WIDTH=52 HEIGHT=88 ALIGN=bottom></CENTER></P> <P><CENTER>You can e-mail me at</CENTER></P> <P><CENTER><A HREF="mailto:myemail@someemail.com">E-mail ME</A> </CENTER></P> <P><CENTER>Or go to this publisher's Web Site </CENTER></P> <P><CENTER><A HREF="http://www.wordware.com">WordWare Publishing</A></CENTER></P> </BODY> </HTML>

You will note a new command at the beginning:
<BODY BGCOLOR="white">

You can change the background color of your page using this command and any standard color. You can also set a background image for your HTML document with a similar command:
<BODY background="mypicture.gif">

The compubook.gif image is stored on the accompanying CD in a folder named examples\chapter 2. If you entered the code properly and used the image located on the CD, your web page should look something like Figure 2-1.

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Figure 2-1. Your first web page.

Now I will be the first to admit that this sample web page is very trivial. But it does contain the basics of HTML. With the material we have covered so far, you can display images, texts, links, e-mail links, background colors, and background images. Not too bad for just a few short pages. You may also want to look at the accompanying CD in the sample HTML folder to see several sample HTML documents for various purposes. These can be used as templates for your own web pages. Examining these can give you a deeper understanding of basic HTML. Let’s examine a few other simple items we can add to our HTML documents. The first is altering text color. You can set the default text color for the entire document and you can alter the color of specific text. You alter the default text color for the entire document using a technique very similar to the one used to alter the background color of the document:
<BODY TEXT="blue">

This text simply tells the browser that unless otherwise specified, all text in this document should be blue. In addition to changing the default color of all text in a document, you may wish to simply change the color of a specific section of text. This is fairly easy to do as well. Instead of using the <BODY TEXT> command we use the <FONT COLOR> command:
<FONT COLOR="red">This is red text</FONT>

This, like the other color commands, can be used with any standard color.

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There are a wide variety of tags you can use to alter the appearance and behavior of texts and images. A few others for you to consider are the <BLINK> and </BLINK> tags which, as the name implies, causes the text to blink (this is only supported by Netscape and will not work in Internet Explorer). Another example is <STRIKE> </STRIKE>, which causes the text to appear with a line through it, a strikethrough. The tags we have covered so far are enough to allow you to accomplish what you need in HTML. The last HTML command we are going to examine is <TABLE>. You frequently see tables on web pages and they are a very good way to organize data on your web page. You can use the tables with or without a border. Here is how you create a table with a border:
<TABLE BORDER=1> <TR> <TD> <P>This </TD><TD> <P>Is a </TD></TR> <TR> <TD> <P>Table </TD><TD> <P>With a border </TD></TR> </TABLE></P>

By now you should be able to recognize that the <TABLE> and </TABLE> tags actually contain the table. Each <TR> tag designates another row in the table. <TD> and </TD> create a cell within that row. Using those three tags you can create a table with any number of rows or columns. Notice in the first line of this code that BORDER is set to 1. This means the border has a width and is therefore visible. In some instances, you may not want the border to show. Tables can be used simply to hold images and text in relative positions. In cases such as these you may not want the border to show. Below is an example of a table whose borders will not show:
<P><TABLE BORDER=0 CELLSPACING=0 CELLPADDING=0> <TR> <TD> <P>This </TD><TD> <P>is a </TD></TR> <TR> <TD> <P>Table </TD><TD> <P>With no borders or padding

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</TD></TR> </TABLE></P>

Notice that BORDER, CELLSPACING, and CELLPADDING are all set to 0. This causes the table itself to not be displayed. However, the contents of the table will display. You should also notice that in both examples there is text in each cell. Figure 2-2 shows a web page with two tables, one with a border and one without.

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Figure 2-2. A web page containing two tables.

Since the purpose of this book is to teach you JavaScript, the obvious question on your mind should be, how do I insert scripts into HTML? Well, fortunately that is not particularly difficult. The browser deals with script code (including JavaScript) like it handles HTML code, scanning the source from left to right and from top to bottom. JavaScript has “tags” of its own by which it instructs the browser and determines the layout. However, JavaScript is not read exactly like HTML. In HTML, the browser acts immediately according to the elements it recognizes. Not all JavaScript code refers to actions that take place while the page is loading. Some parts are just kept in memory until they are called. For instance, if you write a function and do not call it, the browser does not do anything with it. This part of the script stays in memory, and can be invoked later. But the real question you are wondering before you proceed with the rest of this book and learning JavaScript is, how do I put scripts into HTML documents? Any script can be placed very easily by encasing the script you write inside of two commands:
<SCRIPT LANGUAGE = "whateverscriptlanguageyouareusing">

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and
</SCRIPT>

There is a variety of scripting languages available for the web including VBScript (based on the Visual Basic programming language), CGI, and of course, JavaScript. Below is an example of JavaScript inserted into HTML code:
<SCRIPT LANGUAGE="JavaScript"> alert("Wonderful!") </SCRIPT>

Figure 2-3. An alert box.

Let me stress that what I have just covered are simply the essentials of writing HTML code. You can find a number of books that go into HTML in more depth than this. Learn Advanced HTML 4.0 with DHTML by José Ramalho from Wordware Publishing is a good choice if you want to get serious about HTML. There is also a variety of software packages that will do the HTML code for you. You simply create a document just as you would in any word processor and the software creates the underlying HTML code. Two very inexpensive options that I recommend for basic HTML coding are Claris Works Home Page (about $25) or Netscape Composer, which is included with Netscape Communicator.

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Chapter 3

Writing Your First JavaScript
Essential Authoring Tools
In order to begin writing scripts, you need to know which text editor to use. HTML files are plain text (ASCII) files. Therefore, in order to add JavaScript to an existing HTML document, you need a text editor. You should not have any problem getting a text editor for your computer since most operating systems include a text editor (Windows ships with both Notepad and WordPad).

Choosing a Text Editor
You can use various editors, including those described in Chapter 2, for your JavaScript as well as your HTML. However, many experienced web developers simply use the standard text editor that comes with your operating system. If you are using Windows, then you already have Notepad and WordPad on your PC; both are fine for doing HTML and JavaScript. The advantage of using standard text editors is that they are free. The disadvantage is that they do not offer any debugging tools to help you with the JavaScript. The first JavaScript editing environment, Acadia Infuse, was introduced by Acadia Software. It offers an easy-to-use interface and enables fast and easy scripting. You can, of course use simple text editors such as Notepad and WordPad.

Choosing a Browser
Besides the basic programming tool, you need to be able to run your scripts and view their output. In order to run JavaScript, you need a compatible browser. Netscape Navigator 2.0x and up currently support JavaScript. Microsoft Internet Explorer 3.0 and up also support JavaScript (IE’s implementation of JavaScript is called JScript). It doesn’t really matter which browser you choose. The Internet industry is quickly developing, so many new browsers will probably support JavaScript. When you load a local page that includes JavaScript, you do not have to be connected to the net; you can load it directly from your local disk. Overcoming JavaScript compatibility problems is

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often difficult, so I suggest that you have at least one version of Netscape Navigator and one version of Microsoft Internet Explorer.

Basic JavaScript Structure
In order to run client-side JavaScript, you must embed the code in the HTML document. Obviously, you cannot place JavaScript statements in the source code in just any location. There are several different ways to embed JavaScript scripts in HTML: n As statements and functions using the <SCRIPT> tag. n As event handlers using HTML tag attributes. n As short statements resembling URLs.

The <SCRIPT> Tag
Internal Scripts The <SCRIPT> tag is used to enclose JavaScript code in HTML documents. Here is the general syntax:
<SCRIPT LANGUAGE="JavaScript"> [JavaScript statements...] </SCRIPT>

The <SCRIPT LANGUAGE="JavaScript"> tag acts like all other HTML tags. Notice that it must be followed by its closing counterpart, </SCRIPT>. Every statement you put between the two tags is interpreted as JavaScript code. This is probably the most common method for inserting JavaScript into HTML documents. The LANGUAGE attribute is used to specify the scripting language. At present, the <SCRIPT> tag supports various languages including JavaScript and VBScript. JavaScript is the default scripting language, so the LANGUAGE definition is not required. When the browser comes across the name of the language, it loads the built-in JavaScript interpreter and then interprets the script. However, it is always best to explicitly name the scripting language you are using. This ensures that your script will work the same in future versions of HTML and in a variety of browsers. JavaScript is case sensitive, but HTML is not. It does not matter whether you write <SCRIPT> or <script>, but try to be consistent. Personally I use all capitals in my HTML. Even though it does not matter to the browser, it does to the person reading my HTML code. Remember that HTML and JavaScript are no different from any programming language in that you should always strive to make sure your code is readable. External Scripts Netscape Navigator 3.0 introduced a new SRC attribute for the <SCRIPT> tag, which enables the use of external scripts; that is, you can use a JavaScript script that is located in another file. This feature will eventually allow people to hide their scripts from the public, despite the fact that such action is not in keeping with the spirit of the

JavaScript has the same structure, except that only one external file is supported. Here are some disadvantages of using an external file: n No back references. As mentioned above, JavaScript scripts are mainly driven by user-initiated events. A click on a button calls a function, a selection of a form element executes another function, and so on. If you design your whole script in an external file, you will have difficulties in referring to the HTML definitions in the original file. For this reason, place only general functions in an external script. n Additional processing. The JavaScript interpreter evaluates all functions found in your script header, including those defined in the external file, and stores them in memory. Loading unneeded functions in an external file degrades the script performance. n Additional server access. You know how irritating it is to wait until another page loads in the window, especially with slow modem connections. That is the problem with the SRC attribute. When the browser comes across the tag that tells it to interpret JavaScript code in another file, it must first load that file. Such an action is time consuming. Most programmers say that efficiency is not as important as maintainability and clarity. However, with web page development,

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web. I will rarely use external scripts in this book, but remember that they exist, and you may find them helpful. External scripts are useful when you need to integrate a long, sophisticated JavaScript script into an HTML file. This approach has several advantages: n Ease of maintenance. The script is isolated from outside elements, such as non-related HTML code. The maintenance is much easier when the script stands alone. n Hiding from foreign browsers. The code is automatically hidden from browsers that do not support JavaScript. The most basic rule of HTML interpretation, which enables the existence of various browsers, is that unrecognized tags are ignored (try typing <Hello> to your HTML document). So if only the <SCRIPT LANGUAGE="JavaScript"> and </SCRIPT> tags exist, an old browser will ignore all JavaScript-related elements. n Library support. External scripts enable the use of JavaScript libraries, similar to the ones in C, C++, and other programming languages (e.g., #include <string.h>). You can reference general predefined functions in external scripts, thus avoiding duplication of functions or global declarations. Let’s refer once again to C++. Think of cout as a simple function you have written. You must specify the source of the function in order to use it:

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appearance is everything and a slow-loading web site is unlikely to be successful. Always keep in mind that unnecessary HTTP hits to the server should be avoided. Enough theory. Here is the syntax for defining an external script in the <SCRIPT> tag:
<SCRIPT LANGUAGE="JavaScript" SRC="yourFile.js"> [additional JavaScript statements...] </SCRIPT>

Note that the extension of the external file must be .js, just as the extension of an HTML file must be .html or .htm. The name of the file does not have to include the full path; a relative (virtual) path is enough. The .js, obviously, stands for JavaScript. Suppose abc1.js contains the following code:
var counter = 100 function alertMe(message) { alert(message) } function writeMe(message) { document.write(message) }

At this point the meaning of the JavaScript statements themselves is unimportant. They are all explained in the following chapters. Here’s the basic HTML file:
<HTML> <HEAD> <TITLE>Using external files</TITLE> <SCRIPT LANGUAGE="JavaScript" SRC="abc1.js"> <!-var digit = 8 alertMe("Hello!") writeMe("Howdy!") // --> </SCRIPT> </HEAD> </HTML>

3
When you press the OK button you are taken to the image shown in Figure 3-2. Chapter

Figure 3-2. Pressing OK in the alert box takes you to this window.

It is important to understand the way external scripts are interpreted, because many different rules apply to JavaScript and HTML layout. Just keep in mind that an external script is actually interpreted as if it precedes the local script (enclosed by the <SCRIPT> and </SCRIPT> tags). The <!-- and // --> tags are introduced in the next section.

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JavaScript-only files should have the filename suffix .js, and the server must map the .js suffix to the MIME type application/x-javascript, which it sends back in the HTTP header. If the server does not map the .js filename extension to application/xjavascript, Navigator will not properly load the JavaScript file specified by the SRC attribute. In short, your server must be configured to reflect the proper MIME type; otherwise the browser does not respond properly with the data coming back in the HTTP response to the SRC-initiated request.

Hiding the Script
You have probably asked yourself what happens when someone loads your page with an old browser that does not support JavaScript. The solution to this problem is based on the commenting tags of both HTML and JavaScript. Commenting tags in Netscape Navigator and in Internet Explorer are <!-- and -->. The first one opens the comment block, and the second one closes it. A simple comment in a plain HTML document looks like this:
<!-- copyright 2001 -->

There are two types of comments in JavaScript: n // to begin a short comment that does not exceed the length of one line n /* and */ to enclose a comment of any length The JavaScript interpreter ignores the opening HTML comment tag in JavaScript code. Take a look at the syntax you should use to hide the code from old browsers that do not support JavaScript:
<SCRIPT LANGUAGE="JavaScript"> <!-- hide code from old browsers JavaScript statements... // end code hiding --> </SCRIPT>

In order to understand how this hiding works, picture the output of the preceding script in a browser that does not support JavaScript. Both the <SCRIPT> and the </SCRIPT> tags are ignored, like any other unrecognized tag. Everything between the HTML commenting tags is also ignored. This structure works fine with both Netscape Navigator and Microsoft Internet Explorer, which understand the tags and interpret the script correctly.

Problems with Code Hiding
It is important to understand that some browsers may use somewhat nonstandard HTML code. Netscape does this with the commenting tags. While Netscape Navigator expects the sequence <!-- comments -->, standard HTML commenting syntax is <! comments >. Therefore, any greater-than logical operator in the JavaScript source indicates the end of a comment. The script statements beyond this character are interpreted as noncomment plain HTML code, causing the JavaScript statements to be printed to the page as plain text. The only workaround is to avoid using the >

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character in the script. For instance, you can replace b > a with a < b. In some situations, such as printing an HTML tag via JavaScript, it is more difficult to find a replacement for the greater-than character. Take a look at the following script:
<SCRIPT LANGUAGE="JavaScript"> <!-document.write("<TABLE>") // --> </SCRIPT>

An old browser begins to interpret the code as a plain HTML comment when it encounters the <! sequence. It terminates the comment when it finds a > character which is present in the script itself. Therefore, an old browser will print the remaining portion of the script. Generally speaking, you must avoid using a greater-than character literally in a script in order for Netscape browsers to interpret the code properly. The perfect workaround in this case is to assign the greater-than character to a variable without placing it literally in the script, and then evaluating the variable when needed. This would be done as follows:
<SCRIPT LANGUAGE="JavaScript"> <!-var gt = unescape("%3E") document.write("<TABLE" + gt) // --> </SCRIPT>

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Scan the script and notice that no greater-than characters are present. See Chapter 14, Handling Strings, for details on the unescape() function.

Alternate Content
There is even an HTML tag that allows you to provide alternate content for users with browsers that do not support JavaScript. However, you will not see this used very frequently since both Netscape and Internet Explorer support JavaScript in all their latest versions. The <NOSCRIPT> tag and its closing counterpart </NOSCRIPT> are similar to the <NOFRAMES> tag, which is used to provide alternate content to users who are not using a frame-enabled browser. The following example demonstrates how to add such alternative content for users who don’t run JavaScript:
<NOSCRIPT> <B>This page uses JavaScript, so you need to get the most recent version of either Netscape Navigator or Internet Explorer!</B><BR> <A HREF="http://home.netscape.com/comprod/mirror/index.html"> <IMG SRC="NSNow.gif"></A><BR> If you are using Navigator 2.0 or later, and you see this message, you need to enable JavaScript by choosing Options | Network Preferences. </NOSCRIPT>

You might be asking yourself how an old browser that does not even recognize scripts can interpret the <NOSCRIPT></NOSCRIPT> portion correctly. A JavaScript-enabled browser simply ignores that section when JavaScript is enabled. However, an old browser does not know this tag and ignores it. It does not ignore the interior of the

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<NOSCRIPT> and </NOSCRIPT> tags, causing the HTML you put in between to be interpreted only by an old browser or one that has its JavaScript support disabled.

Placing JavaScript Code
When you want to place the script somewhere in the HTML document, you need to choose where to put it. Technically, you may place it anywhere between the <HTML> and </HTML> tags that enclose the whole document. Actually, the two possibilities are the <HEAD></HEAD> portion and the <BODY></BODY> portion. Because the <HEAD></HEAD> portion is evaluated first, some developers choose to place their JavaScript here. A single HTML document may contain any number of scripts. You can place some of the scripts in the <HEAD></HEAD> portion and others in the <BODY></BODY> portion of the page. The following code demonstrates this:
<HTML> <HEAD> <TITLE>Multiple scripts</TITLE> <SCRIPT LANGUAGE="JavaScript"> [JavaScript statements...] </SCRIPT> <SCRIPT LANGUAGE="JavaScript"> [JavaScript statements...] </SCRIPT> </HEAD> <BODY> <H1>This is a complex structure</H1> <SCRIPT LANGUAGE="JavaScript"> [JavaScript statements...] </SCRIPT> </BODY> </HTML>

Conventions
JavaScript syntax is very similar to C, C++, and Java. It includes functions, expressions, statements, operators, and more.

Using the Semicolon
The JavaScript interpreter does not pay any attention to carriage return characters in the source. It is possible to put numerous statements on the same line, but you must separate them with a semicolon (;). You can also add a semicolon at the end of a statement that occupies its own line, but it is not necessary. Take a look at the following statements:
document.write("Hello"); alert("Good bye") document.write("Hello") alert("Good bye")

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document.write("Hello"); alert("Good bye");

All three sets are legal, and their results are identical.

JavaScript is Case Sensitive
You saw earlier that JavaScript is a case-sensitive language. This applies to all aspects of the language, including variable names (identifiers), functions, and methods (discussed later). The statement document.write(), for example, is legal, but document.Write() is not.

Comments
Comments are an important concept in all languages, including in JavaScript. They help make programs simple and easy to understand. Comments are messages that can be put into programs at various points, without affecting the results. There are two different types of comments in JavaScript: n Single-line comments are comments that do not exceed the length of a line. They must start and end on the same line. These comments begin with a double slash (//). n Multiple-line comments are comments that may exceed the length of one line. Therefore, they require a sign that specifies the beginning of the comment, and another sign that specifies the end of the comment. The opening part is /* and the closing part is */. Both types of comments are identical to the comments in C and C++. You should use comments to explain every part of your script.
// single-line comment /* line 1 of the comment line 2 of the comment line 3 of the comment */

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Using Quotes
In JavaScript, you often use quotes to, among other things, delimit strings. A common problem arises when using a pair of quotes inside another pair of quotes. Since the interpreter must recognize each set of quotes in order to pair them correctly, the creators of JavaScript made it possible to use two different types of quotes: double quotes (") and single quotes ('). If you need only one set of quotes, you can choose either of them as long as you terminate the quote with the same type of quote you used to open it. If you do not follow this rule, you will get a JavaScript error: “unterminated string literal.” You must always alternate quotes properly:
document.write("<IMG SRC='cool.gif'>")

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Your First Script
By now you may be asking yourself, when do I actually write some JavaScript? The answer to that question is right now! I have discussed with you the theory behind JavaScript until you are undoubtedly tired of reading it. So now let’s move on into some actual script. First of all, launch your text editor. Type Example 3-1 in the text editor and save it under the name HelloWorld.htm. Make sure the name ends with either the .htm or .html extension or your browser will not recognize this as an HTML document. It is imperative that you make every attempt to fully understand what it is you are typing as you type in this example. Next, launch the browser. Since the file is local, you do not need to be connected to the Internet to view the results. Now, load the file from the browser’s menu. That’s all there is to it. You can start enjoying JavaScript. The following script is interpreted and executed immediately when you load the page containing it.
<HTML> <HEAD> <TITLE>Hello World.</TITLE> </HEAD> <BODY> <SCRIPT LANGUAGE="JavaScript"> <!-- hide code from old browsers document.write("<H1>Hello World.</H1>") // end code hiding --> </SCRIPT> </BODY> </HTML>

Example 3-1

If you entered the code properly you should see something in your browser similar to Figure 3-3.

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This is a simple HTML document that includes a JavaScript script. Admittedly, it is nothing to get overly excited about. However, it does illustrate exactly how to place JavaScript into an HTML document. Notice that its structure is the same as that of any other HTML document. The only new concept is the <SCRIPT> tag. I put the script in the <BODY>…</BODY> portion of the page, though you may put it anywhere between the <HTML> and </HTML> tags. For now, think of document.write() as a way to print expressions to the page. write() is actually a method of the document object. Objects, methods, and properties are introduced in Chapter 5, Object-Oriented Programming. The write() method supports any HTML syntax. Be aware, also, that all strings must be included in quotes. Example 3-2’s equivalent consists of two files, the HTML file and the external JavaScript file:
<HTML> <HEAD> <TITLE>Hello World.</TITLE> </HEAD> <BODY> <SCRIPT LANGUAGE="JavaScript" SRC="sayHello.js"> </SCRIPT> </BODY> </HTML> // this file must be named sayHello.js document.write("Hello World.")

If you enter this code properly, you should see something in your browser similar to Figure 3-4.

Figure 3-4. Printing the script output.

In order to understand the printing concept, it is necessary to compare the screen output with the paper output.

Updating a Page
Whenever you change the content of the page and you wish to see how it affects the final layout, you must reload the page using the Reload button. A JavaScript script is immediately interpreted upon initial loading of the page, and code that does not run

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immediately is stored in memory. Reloading the page refreshes the memory and generates a new version of the page layout (if you changed it, of course).

Summary
In this chapter you learned how to write scripts and add them to an HTML document. We covered the <SCRIPT> tag and other related topics. It is sometimes difficult to predict what other people see on your page, especially those who are using a browser that does not support JavaScript. Although you haven’t seen any useful JavaScript script yet, we hope you understand that this is a powerful scripting language that can do much more than generate HTML content with document.write(). Once you get a grip on writing simple scripts, you will find it easy to move on to more complicated and interesting programs. It may not have seemed like much, but you have laid the foundation for learning JavaScript.

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JavaScript Fundamentals
“Building” a Script
JavaScript has many fundamental concepts in common with all programming languages, including its basic elements. It has variables which hold data, it has expressions or statements, and it has functions. These building blocks are used to construct your JavaScript. In this chapter I will explain each of these elements.

Data Types in JavaScript
Variables hold information which must be of a specific type. These types are referred to as data types. It is important to use the proper date type to store your information. For example, a number is a type of information that JavaScript recognizes. Compared to most programming languages, JavaScript has a small number of data types, but its methods, objects, and other elements help overcome this limitation. There are four data types in JavaScript: numbers, strings, Boolean, and null values. As opposed to other languages, a variable data type is not declared explicitly but rather implicitly according to its initial value assignment. Also unique to JavaScript, there is no explicit distinction between integer and real-valued numbers. All of these data types are specified in Table 4-1.
Table 4-1. Data types in JavaScript. Type Number String Boolean Null Description Any number without quotes A series of characters enclosed in quote marks A logical value A keyword meaning no value Examples 42 or 16.3 or 2e-16 "Hello!" or "10" or ' ' or "" true or false null

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The Value Null
The value null is often used to initialize variables that do not have any special meaning (see “Variable Declaration” later in the chapter). You assign it to a variable using the standard assignment statement:
var name = null

The null value is special in that it is automatically converted to initial values of other data types. When used as a number it becomes 0, when used as a string it becomes "", and when used as a Boolean value it becomes false. Since the source of many JavaScript errors is uninitialized variables, one of the common debugging techniques is to initialize all uninitialized variables, including the meaningless ones, with a null value. The JavaScript interpreter uses the null value on two occasions: (1) built-in functions return null under certain circumstances, and (2) non-existent properties evaluate to null. When checking for a null value, you should check if it is false or if it is equal to null.

Variables
Variables are the cornerstone of most programming and scripting languages. They serve as a link between simple words in your script and the computer allocated memory. There is a limit to the amount of memory you can use, but you should not reach that limit. JavaScript applications are not heavy resource demanders and modern PCs typically have a great deal of memory, so exceeding available memory is probably an indication of a bug in the browser or a major flaw in your program (such as an unexpected infinite loop). Because you do not deal directly with the memory allocation, you should think of variables as baskets that contain items. You can put an item in a basket, take it out, or replace it with another. A script that does not use variables is probably useless. It’s like picking strawberries and taking them home in the palm of your hand, rather than using a basket—impossible! You need variables to keep data of different types.

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Identifiers
Each variable is identified by a variable name, also known as an identifier. Each variable name is associated with a specific memory location, and the interpreter uses it to determine its location. There are strict rules for naming variables: n The first character of an identifier must be either a letter (uppercase or lowercase) or an underscore (_). n All other characters can be letters, underscores, or digits (0-9). n An identifier cannot be one of the language’s reserved words. Reserved words consist of all JavaScript keywords as well as other tokens reserved for future versions.

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An identifier length is not limited, and you should take advantage of this feature to select meaningful names. JavaScript is case sensitive (uppercase letters are distinct from lowercase letters). That is, counter, Counter, and COUNTER are names of three different variables. Avoid using such similar identifiers in the same script. However, you will want to use variable names that are related to the data they contain. For example, if you have a string variable that holds a person’s last name, you might wish to name it LastName.

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Caution: The $ character is not legal in some older versions of Netscape and Internet Explorer. For example, the following identifiers are accepted by Netscape Navigator 3.0x and 4.0 but not by 2.0x and Internet Explorer 3.0x.
$FirstDigit money$ tenth$cell$10

The following identifiers are legal:
gameCount _hamburger _123456789_ look_at_me Number16

but the following ones are illegal:
with ^fastTimer 911phoneNumber 04-825-6408 // // // // // ***important*** // 10_guesses // reserved word first character is illegal cannot start with a digit first character is illegal "-" is an illegal character * is not a legal character first character cannot be a digit

Keywords are words that have special meanings in a programming language. You cannot use keywords to name variables or functions you create in a script (such as variables, functions, etc.). The list of keywords is the basic vocabulary of the language. The word if, for example, is a keyword. You do not have to memorize the list, because you will gradually remember it as you use the words in your scripts.

Naming Conventions
There are a number of generally accepted conventions in JavaScript: n A variable name is normally written in lowercase letters. n The variable name indicates its purpose and use in a program. n In a multiword identifier, either place an underscore between words or capitalize the first letter of each embedded word.

Avoid similar variable names. The following illustrates a poor choice of variable names:
digit digits // current digit // number of digits in the number

A much better set of variables is:
current_digit num_of_digits // current digit // number of digits in the number

Variable Declaration
Before you use a variable, you need to create it. JavaScript is a loosely typed language, which means that you do not have to explicitly specify the data type of a variable when you create it. As needed, data types are converted automatically during the course of the script execution. In strongly typed languages the variable must be created of a specific type. There are two ways to create a variable. The first type of declaration includes the var keyword, followed by the name of the variable:
var variableName

The action of assigning an initial value to a variable is called initialization. You give the variable a value using the most common assignment operator—the equal sign:
var variableName = initialValue

You only need to use the var keyword when you create the variable. When you want to refer to the variable, you only use its name. Assign a value to a variable (after it has been declared) in the following fashion:
variableName = anyValue

You use var only once per variable. A global variable can be created simply by assigning it a value without the var keyword. Local variables inside functions, on the other hand, must be declared with the var keyword. As in many other programming languages, JavaScript allows you to declare numerous variables in the same statement, using a comma to separate them:
var variableName1 = initialValue1, variableName2 = initialValue2, …

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When interpreting this statement, the browser creates a link between the name of the variable and its memory address, so successive references can be done by name. Unlike some programming languages, declarations are not limited to a specific zone but can be done anywhere throughout the script.

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JavaScript Entities
JavaScript entities can be assigned to HTML attributes. This attribute substitution enables creation of more flexible HTML constructions, without the writing overhead of full JavaScript scripts. Note that JavaScript entities are not supported by older versions of Netscape and Internet Explorer. You are probably familiar with HTML character entities with which you can display a character by its numerical code or name. You precede a name or a number with an ampersand (&) and terminate it with a semicolon (;). Here are a few examples:
&gt; &lt; &#169;

The entity &{fontColor}; is replaced by the current value of fontColor and &{fontSize}; is replaced by the current value of fontSize. Since JavaScript can only use values that are stored in memory at the time of page layout, JavaScript entities should be used only after calling the script that assigns their value.

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Once layout is complete, the display of the page can change only if you reload the page. Even if the value of the JavaScript entity variable changes, the entity itself does not change until you reload the page. Unlike HTML character entities which can be used in any script statement, JavaScript entities can be used only in a tag statement. Another difference is that, while HTML character entities may substitute for any HTML element, JavaScript ones are limited to HTML attribute substitution only. For example, you cannot specify the entity "&{text};" with the variable text = "<H1>Hi!</H1>"—it is not a valid value for a tag attribute.

Type Conversion
As mentioned above, data types are converted automatically as needed during the course of script execution. A variable may hold a numeric value at one point of the script and a string at another. The following statements constitute a valid JavaScript script:
var myVar = 12 myVar = "university"

The first statement assigns a numeric value to myVar, and the second one assigns it a string. Such conversions are not allowed in strictly typed languages such as C++ and Java. While they are possible in JavaScript, they are not recommended. Using this technique makes for very sloppy programming.

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Mixing Strings and Numbers
Mixing strings and numbers is sometimes necessary for certain operations. Since this is tricky and can generate unexpected results, you should be familiar with its exact rules. When an expression including both numbers and strings is evaluated to a single value, that value evaluates to a string. Converting it to a number is usually impossible. For example, the number 6 can be easily converted to a string (6), while the string “horse” cannot be converted to a number. Another important rule is that the JavaScript interpreter evaluates expressions from left to right, and only parentheses can change the order of evaluation. Take a look at the following expressions, numbered by lines.
/* /* /* /* /* /* 1 2 3 4 5 6 */ */ */ */ */ */ 8 + "8" 8 + "8" 8 + 8 + 8 // 16 + 8 // "88" "8" // "88" + "8" // "88" 8 + "8" // "168" "8" + 8 // "888"

These expressions all use the string concatenation operator which is also the numeric plus operator. (Operators are covered later in this chapter.)

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The first expression simply adds up two numbers using a numeric format. It uses the standard plus operator and evaluates to the sum of its operands. The second expression is quite different. Its first operand is a string rather than a number. In order to add a number to a string, the number is converted to its matching string and the strings are then concatenated. The third and fourth expressions are similar in that they also include at least one string operand. The fifth expression is a bit trickier. The first two operands are added up because they are numbers. The expression now evaluates to 16 + "8" which, as you already know, evaluates to "168". Using parentheses can change the result. For example:
8 + (8 + "8") // "888"

In this expression the content in the parentheses is evaluated first to "88" and the entire expression evaluates to "888". Comparing the sixth expression to the fifth one clearly demonstrates what left-to-right evaluation means. Whenever a string is found during the expression evaluation, the accumulated value thus far is converted to a string. The remaining numeric values on the right-hand side are automatically converted to strings when concatenated to this accumulated value. If you want to convert a single number to a string, you can use one of the following three methods:
var newVar = " " + numericValue var newVar = new String(numericValue) var newVar = numericValue.toString()

The first method is simple. You use the concatenation operator, which instructs JavaScript to convert the number to a string and then append it to a null string. The other methods are discussed later in the book.

Literals
Literals are fixed values that you literally provide in your application source, and that are not variables or any other data structure. They are notations for constant values and therefore do not change throughout the script. A literal gives you a value instead of merely representing possible values. Unlike C++ and Java, which have five literal types, JavaScript has only four: integer, floating point, Boolean, and string. Character literals are simply considered strings with a length of 1.

Integer Literals
Integer literals, also called whole numbers, are numbers that have no decimal point or fractional part. Here are some integer literals:
49 16 0 –18 –42

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An integer can be positive, negative, or zero. JavaScript, like most other programming languages, supports integers of three types or bases: decimal, octal, and hexadecimal. Decimal Integers Decimal integers, also known as base-10 integers, are the common integers we use daily. They are written with the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. Except for the number 0 itself, a decimal integer cannot have a leading 0 digit. It makes no sense to write the current year as 01997, and, indeed, JavaScript will not evaluate it as 1997. Octal Integers Octal integers, also known as base-8 integers, use only eight digits: 0, 1, 2, 3, 4, 5, 6, 7. Octal digits are written with a leading 0 digit (that’s a zero, not the letter “o”). If you want to reference the octal integer 12 (equal to 10 in the decimal system), you would have to write it as 012. Hexadecimal Integers Hexadecimal integers are commonly used in programming environments because each hexadecimal digit represents four binary bits. A hexadecimal number is any sequence of these digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F. The integers are called hexadecimal because they are based on 16 different digits. Case sensitivity rules do not apply to numbers, so you can use lowercase letters as well. The prefix for hexadecimal numbers is 0x or 0X. Hexadecimal numbers might look familiar to HTML authors. In early versions of Netscape Navigator and Internet Explorer, colors were specified in a hexadecimal triplet format. Although the latest versions of Netscape Navigator and Internet Explorer let you specify colors by their names, some people continue to use the hexadecimal notation. The following tag sets the background color to the default gray (even for users who changed their default to white):
<BODY BGCOLOR="#c0c0c0">

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You probably know that colors differ in the relative contributions of red, green, and blue. The hexadecimal triplet combines three hexadecimal numbers of two digits each (2 digits = 16 * 16 = 256 possibilities). Converting from Decimal to Other Bases You can use a calculator to convert a decimal integer to a different base. But what happens when the calculator is broken?! It is surprisingly simple to do the conversion with a pencil and a piece of paper. You start by dividing the decimal number by the target base (e.g., 2 for binary, 8 for octal, 16 for hexadecimal). Write down the remainder. Now do the same with the quotient, writing the new remainder to the left of that from the previous operation. Keep looping until the quotient is less than the target base. The following table shows the conversion of the decimal number 747 to a hexadecimal notation:

Take a look at the table’s far right column. Reading it bottom up, we get 2EB16, which is exactly 74710. The same process converts decimal integers to any base. Converting from a Specified Base to Decimal It is equally easy to convert a number from a specified base to a decimal notation. First, let’s analyze a number in decimal notation, say 276: 276 = (6 * 100) + (7 * 101) + (2 * 102) = 6 + 70 + 200 Each digit contributes the value digit * basedigitPlaceFromRight, where digit is the current digit, digitPlaceFromRight is the digit’s position (starting at the right-hand side 0 position), and base is the current base. The number is equal to the sum of all contributed values. Conversion from a given base to a decimal notation uses the same technique, except that each digit is worth its value in decimal notation. Let’s convert 14728 (octal) to a decimal base: (2 * 80) + (7 * 81) + (4 * 82)+ (1 * 83) = 2 + 56 + 256 + 512 = 82610 We use these techniques to create automatic conversion functions later in the book. Referring to Octal and Hexadecimal Integers Octal integers have a leading 0 character. Hexadecimal integers have a leading 0x or 0X prefix. Decimal integers are normal integers with no prefixes. So how do we work with these prefixes? JavaScript refers to all integers as decimal. So if you write 0x2EB, JavaScript stores the number and refers to it as 747 in decimal notation. The same rule applies also to octal integers. They are stored, displayed, and handled according to their decimal value. The computer obviously stores them as binary numbers, but to the JavaScript programmer, it seems as if they are handled in decimal form.

Floating-Point Literals
Floating-point numbers (sometimes called real) have a fractional part. JavaScript uses a decimal point to distinguish between floating-point numbers and integers. Literally, a number such as 16.0 is a floating-point number, whereas 16 is an integer. Additionally, a floating-point number may include an exponent specification of the form e±exp. Since there is no explicit declaration of data types in JavaScript, the floating-point format is implicitly determined by the literals that are assigned to the variables. Here are some floating-point numbers, with some explanations:

Notice that the exponent specification can be in uppercase or lowercase characters. Floating-point math is much more difficult than integer math and is the source of many bugs. Overflow describes the situation in which the exponent of a floating-point number exceeds the upper limit. Underflow occurs when the exponent is below the lower one. You should avoid calculations that are likely to result in an overflow or an underflow. A more common problem with floating-point numbers is the roundoff error. Everyone knows that 5 + 5 is 10, but is 1/3 + 1/3 equal to 2/3? The answer is: not always. Take a look at the following to find out why: 1/3 as floating point is 3.333E–1 (rounded off). 2/3 as floating point is 6.667E–1 (rounded off). 3.333E–1 + 3.333E–1 = 6.666E–1 As you can see, the result is different from the normal value of 2/3 because 6.666E–1 6.667E–1. Every computer has a similar problem with floating-point values. The fraction 0.2, for example, has no exact binary representation. For this reason, always round off your floating-point values before doing calculations in order to avoid these inaccuracies.

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Boolean Literals
Boolean values, also called logical values, are basically true or false. They are usually used in conditional expressions and statements. If you are familiar with C++, you probably recognize these terms as 1 and 0, 1 representing true and 0 representing false. JavaScript officially uses the true and false values to express Boolean values, but 1 and 0 are acceptable in most situations. The true value, or 1, can usually be replaced by any nonzero integer. Avoid using these numeric values as Boolean, because Netscape has not officially recognized this kind of usage and may opt to invalidate it in future versions of the browser. Usage of integer values to represent Boolean values, as in other programming languages, can cause data type confusion and should not be done at all by novice programmers.

String Literals
String literals are delimited by either single or double quotes. You must terminate the string with the same type of quote you used to open it, so "Hi' is not a legal string in JavaScript. Unlike strings in most programming languages and shells, JavaScript does

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not distinguish between single and double quotes. They serve exactly the same purposes. Strings in JavaScript are not subject to variable interpolation, i.e., you cannot embed variables directly in the string and expect them to be replaced by the value they hold. Perl, for example, features variable interpolation (or variable substitution) because variables can be identified by the preceding $ character. The alternative delimiters q/string/ and qq/string/ are not supported in JavaScript. You must always use the traditional quotes to delimit strings. Nested strings are widely used in JavaScript. A nested string consists of a string inside another one. Alternating quotes enables proper interpretation of nested string constructions. The following statement demonstrates how to alternate quote types:
document.write("<FONT COLOR='red' SIZE=4>")

The document.write() statement requires quotes, and so does the COLOR attribute of the <FONT> tag. You may use single quotes for the string red and double quotes for the longer enclosing string. You may also use escape quotes, as explained below. Be careful to place the trailing quote at the end of the string. If you forget it, the relevant error will be reported only after JavaScript runs into the end of the file or finds a matching quote character in another line. Fortunately, such errors will be detected immediately as syntax errors on the following line. Sometimes, though, the error message does not give any clue about the runaway string, so paying extra attention when dealing with strings is a good strategy. Strings often include escape sequences, also called escape characters, special characters, or control characters. Such sequences have special purposes. For example, \t is the tab character. They are usually used to express nonprintable characters or other problematic ones. The following table outlines the escape sequences:
Table 4-3. Escape sequences in JavaScript. Escape sequence \ddd* \xdd* \\ \' \" \b \f \n \r \t \ Character 0ddd 0xdd \ Meaning octal character hexadecimal character backslash single quote double quote backspace form feed new line (or line feed) carriage return horizontal tab continuation

' "
BS FF NL or LF CR HT <new line>

* The d character represents a digit.

Be sure to use these inline characters only where needed. You should use the standard HTML tags for line breaks (<BR>) and paragraph breaks (<P>). The carriage return

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escape sequence creates a new line only in dialog boxes and text area objects. Most of the escape sequences act the same way. Do not expect them to format the look of the page, because layout relies solely on HTML. Here are some strings that take advantage of escape sequences:
"\x2499.99" // $99.99 (the hex value of the $ char is 24) 'c:\\games\\sc2000\\' // c:\games\sc2000\ 'Let\'s learn JavaScript...' // Let's learn JavaScript... "line1\rline2"

To see the effect of a carriage return character, try displaying the last string in an alert box.

Operators
Every programming language has operators. An operator is simply a symbol that tells the compiler (or interpreter) to perform a certain action. The basic arithmetic operators are common to most programming languages. These are addition (+), subtraction (–), multiplication (*), and division (/). These should be very familiar to most people. The order of precedence of operators follows the standard mathematical rules of multiplication, division, addition, and subtraction. However, when your code has multiple operations in a single line, it is usually a good idea to use parentheses to clarify what you want to occur: 3 * 4/2 + 1 can be ambiguous, whereas 3 * ( (4/2) + 1) is very clear. C, C++, and Java programmers will already be familiar with the increment and decrement operators. The increment operator is formed by placing two plus signs after a variable, such as this:
var somenumber somenumber++

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This line of code increments the value of somenumber by one. Had we written:
somenumber - -

It would have decreased the value by one. It is very important that you realize that where you place increment and decrement operators is critical. If you place the increment operator after a variable such as:
var somenumber = 10 var someothernumber someothernumber = somenumber++

The assignment operation will take place before the evaluation operation. In other words, first someothernumber will be set equal to the value of somenumber, then the value of somenumber will be incremented. In our example that means that someothernumber will equal 10 and somenumber will equal 11. If you wish to rewrite the statement so that the increment takes place first, just reposition the increment sign:

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someothernumber = ++somenumber

In this case, somenumber is incremented to 11 and then that value is assigned to someothernumber. You’ve already learned how to assign a value to a variable or to initialize it, using the equal assignment operator. As the following piece of code demonstrates, you can also perform calculations when you assign a value:
/* 1 */ var answer /* 2 */ answer = 4 * 2 + 9 /* 3 */ document.write(answer)

Line 1 includes the declaration of the variable answer. The second line shows how the variable answer is assigned the result of a simple mathematical expression. At this point, the variable holds a value of 17. Referring to the variable answer is the same as referring to the number 17. For this reason, the statement on line 3 prints the number 17.

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Caution: A very common mistake is to use the equal sign for equality check. In Pascal, for example, “=” is an equality test operator because the basic assignment operator of the language is “:=”. However, in JavaScript, like in C++ and Java, “=” (the equal sign operator) is an assignment operator, while “==” (two equal signs) is an equality test operator.

This is just a cursory examination of the simplest operators in JavaScript. A thorough discussion of operators is given in Chapter 7. However, your understanding of this chapter and the next two chapters depend on a brief introduction to operators.

Making Statements
Now that we have thoroughly examined data types, let’s look at statements. A statement is a line of code that performs some specific task. For example, all of the following are statements:
yourAge = 32 for(x=1;x<10,x++) myname= “Chuck”

Multiple Statements
The JavaScript interpreter accepts multiple statements on the same line. If you choose to use this method, you must separate the statements with semicolons (;). The last statement of the line does not have to be followed by a semicolon. Such a line looks like this:
statement1; statement2; statement3; ...

The browser interprets these statements as if they were listed on separate lines:

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statement1 statement2 statement3

Although this is possible with JavaScript, I certainly do not recommend it. Placing multiple statements on a single line makes for very unreadable code. Even though JavaScript does not require semicolons after each statement (unless they are on the same line), it does not generate an error if you place them at the end of each statement. So if it is one of your programming habits, you may terminate all statements with semicolons.
statement1; statement2; statement3;

Remember, however, this is not required in JavaScript as it is in C, Java, and C++.

Nested Statements
A command block is a unit of statements enclosed by curly braces. It is very important to understand that a block should be used as a single statement. The statements inside the block are called nested statements:
{ nested statement1 nested statement2 nested statement3 } command block (one statement)

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A loop that includes many statements is actually one statement with many nested statements. This rule applies to functions, if-else statements, and other language elements.

Evaluating Expressions
Now that you know how to create a variable, you need to know how to use it. As mentioned earlier, variables hold values of different types. What does “holding a value” mean? This term refers to expression evaluation. A variable always evaluates to its value. When you perform an operation on a variable, you are actually performing the operation on the current value associated with the variable. Let’s assume you created a variable named firstNumber using the following statement:
var firstNumber = 120 // declaration and initialization

At this point, if you refer to the variable firstNumber, its value, 120, is returned. That is, firstNumber is evaluated to 120. The following statement outlines an evaluation of firstNumber:
secondNumber = firstNumber * 6

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The secondNumber variable now holds the value 720, because firstNumber evaluates to 120. Bear in mind that no link between the memory locations of the variables is established. Therefore, secondNumber now holds a value of 720, which does not change even if the value of firstNumber changes. A variable can evaluate to a value of any type (see Table 4-1).

Testing Evaluation
A hidden feature of Netscape Navigator (but not of Internet Explorer 3.0x) enables you to experiment with evaluation in the browser’s window. You can reach this window by simply entering javascript: in the Location box. Another way to do this is by choosing Open Location from the File menu. Then type javascript: to open the evaluation window. The evaluation window contains two frames. The field in the bottom frame is used to accept your input and the upper frame displays the Navigator’s computation results. To experiment with this tool, enter the following statements at the javascript typein field at the bottom:
var firstNumber = 120 var secondNumber = firstNumber – 60 firstNumber secondNumber var finalAverage = (firstNumber + secondNumber) / 2 finalAverage secondNumber = firstNumber finalAverage finalAverage > secondNumber

Before you enter these expressions, try to figure out what they evaluate to. Then type them in the field in the bottom frame, with a carriage return (Enter) after each statement.

Functions
Function Definition
You have already seen that a function is actually a multiline statement that includes many nested statements. Just like variables, you must define a function before you can call it. Functions are defined using the keyword function, followed by the name of the function. The same rules that apply to variable names apply to functions. Since a function usually does something besides storing a value, it is common to include a verb in its name. The function’s parameters are written in brackets after the name. A command block follows the parameters. The syntax of a function definition is:
function functionName([parameters]) {

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[statements] }

Parameters are local variables that are assigned values when the function is called. At this point, you should always give a name to every parameter. In a formal syntax specification, the square brackets ([]) usually denote optional elements. Since a function does not have to have parameters or statements, they are both enclosed in such brackets. The curly braces enclosing the function body can be placed anywhere, following the parameters’ section. The following functions are valid:
function functionName([parameters]) {[statement1]; [statement2]; …} function functionName([parameters]) { [statement1] [statement2] }

Example 4-2 does not print anything to the browser’s window, nor does it generate any other form of output. The reason is that the function is only defined in the script but is never called. When the browser locates a function, it loads its statements into memory, ready to be executed later.

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Calling Functions
In order to execute the set of statements located in the function block, you must call the function. The syntax of a function call is:
functionName([arguments])

By adding the statement square(5) to Example 4-2, at the specified place, we call the function. The statements in the function are executed, and the following message is output:
The call passed 5 to the function. 5 square is 25.

Example 4-3 summarizes many of the terms discussed in this chapter. It includes two function definitions. In both functions, the output is assigned to a variable (output) and then printed to the client window using the document.write() method. Assigning strings to variables before printing them is extremely useful when the string is long (i.e., you want to print a lot of data). You can see the result of Example 4-3 in Figure 4-1.

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Figure 4-1. HTML content generated by a function.

Control Structures in JavaScript
Dialog Boxes
Before we discuss the control structures in JavaScript, we need some basic userinteraction devices. These will allow us to create both useful and helpful examples for demonstration purposes. Dialog boxes are only introduced in this chapter. They will be discussed later in further detail. JavaScript provides the ability to create small windows called dialog boxes. You can create alert boxes, confirm boxes, and even prompt boxes. These boxes let you generate output and receive input from the user. Alert Boxes An alert box is the most simple dialog box. It enables you to display a short message to the user in a separate window. Take a look at the following script and its corresponding output:
alert("Click OK to continue...")

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The generic form of this function is alert(message). The function alert() is actually a method of the window object. It is not necessary to specify that because window is the default object. The same applies to all dialog boxes.

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þ

Note: Netscape Communications Corp. implemented the “JavaScript Alert:.” header for security reasons. It is used to distinguish JavaScript dialog boxes from those created by the operating system, so that the user knows what the source of the message is. JavaScript programmers cannot trick the user into doing something he might not want to do. It also disables the ability to scare the user into giving away personal information. According to JavaScript developers at Netscape, the design of these dialog boxes might change in the future, but Netscape Navigator 2.0x, 3.0x, and 4.0 generate the box in the format described. Note that Internet Explorer dialog boxes do not include this warning. In either case, however, a dialog box will look essentially like the one shown in Figure 4-2.

Figure 4-2. A typical dialog box.

You can also display messages using data structures. For example:
var message = "Click OK to continue" alert(message)

As you can see, the alert box is often used to pause the execution of a script until the user approves its continuation. Confirm Boxes Confirm boxes are different from alert boxes in that they evaluate to a value based on a decision made by the user. Rather than a simple OK button, the confirm box includes both OK and Cancel buttons. Like the alert box, confirm is also a method of the window object. This method returns a Boolean value, because there are two options. You can use confirmation boxes to ask the user a yes-or-no question, or to confirm an action. Here is an example and its output:
var reply = confirm("OK to continue?")

reply is assigned a true value if the user chooses OK, and false if the user selects Cancel. The generic form of this function is confirm(message).

Prompt Boxes
The prompt() method displays a prompt dialog box with a message and an input field. You can use these boxes to receive input from the user. It is similar to the confirm box, except that it returns the value of the input field, rather than true or false. Here is an example:

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var name = prompt("Enter your name:", "anonymous")

The method returns a value of null if the user chooses Cancel. The prompt box looks like the image shown in Figure 4-3.

Figure 4-3. A prompt box.

The value of the field is always a string. If the user enters 16 in the form, the string "16" is returned rather than the number 16. When you want to prompt the user for a number, you must convert the input into a numeric value. JavaScript features a built-in function that does this—parseInt(). You can use the following statement to ask the user for a number:
var number = parseInt(prompt("Enter a number:", 0))

or
var number = prompt("Enter a number:", 0) number = parseInt(number)

The generic form of this function is prompt(message[, inputDefault]). You can see that this function works by using the typeof operator for testing:
var number = prompt("Enter a number:", 0) alert(number, " is a ", typeof number) // "... is a string" number = parseInt(number) alert(number, " is a ", typeof number) // "... is a number"

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The input must be of a numeric type, of course (e.g., 99).

if Statement
if (condition) statement

The if statement lets you put decision making in your scripts. A script without any decisions does the same procedure each time it is executed. Such linear structures limit your scripts to simple algorithms. JavaScript enables decision making using an if statement. if statements associate a single statement with a true condition. That statement is only executed if the conditional expression is true; otherwise, it is not executed at all. The condition must evaluate to a Boolean value: true or false. Numeric values are also acceptable as an alternative to a Boolean condition. 0 is equivalent to false, and all other values are equivalent to true. The if statement associates a single statement with a true condition. A statement can be anything from a simple document.write() to a block of statements, using curly

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braces ({}). Some if statements require multiple statements, so they use a block in the following form:
if (condition) { statement1 statement2 statement3 }

A nested statement can be any legal statement, including an additional if statement. Here is a simple example demonstrating the if statement:
<HTML> <HEAD> <TITLE>A simple if statement</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-var age = parseInt(prompt("Please enter your age:", 15)) if (age < 21) alert("Sorry, you are too young to enter") // --> </SCRIPT> </HEAD> <BODY> </BODY> </HTML>

Example 4-4 (ex4-4.htm). A script with one conditional statement.

At first, the script asks the user for his or her age.

Figure 4-4. A prompt box that requests the user’s age.

The age is stored in numeric format in the variable age. The if statement checks if the user’s age is less than 21. If so, the expression age < 21 evaluates to true. Because the condition is true, the next statement is executed, and an alert box is displayed. Note that if the value of age is greater than or equal to 21, no statements are executed. The remedy to this problem is presented in the next section. Here is another example using multiple statements associated with a true condition:
<HTML> <HEAD> <TITLE>An if statement with a command block</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!--

If you enter this code properly into your editor and view the web page in either Netscape or Internet Explorer, you should see something similar to Figures 4-5 and 4-6.

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Figure 4-5. A prompt box that requests the user’s name. Figure 4-6. A confirm box verifying the browser version number.

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Example 4-5 features an if statement with a set of statements grouped together in a command block. Notice the function that prints a horizontal rule of a specified width. In this example, the user must reply to two dialog boxes. If the user responds by clicking Cancel to the confirm dialog box, the page looks like this:

You probably felt limited with the structure of the if statement, because it only lets you execute a statement when the condition is true. The additional else statement specifies a statement that is executed when the condition is false. This construction covers all possibilities, because a condition can be either true or false. Here is a script segment extracted from Example 4-4, improved with an else statement:
var age = parseInt(prompt("Please enter your age:", 120)) if (age < 21) alert("Sorry, you are too young to enter") else alert("Welcome in...")

If the user’s age is greater than 21, he receives a welcoming message.

The function accepts a character (a one-character string) as its argument. If the character is greater than A and less than Z, it must be an uppercase letter, and an appropriate message is displayed. Only if the character is not an uppercase letter does the execution continue. If it is a lowercase letter, the appropriate message is provided. Once again, only if it is not a lowercase letter (meaning it is not a letter at all) does the execution proceed. No more tests are performed, so the last message is generated. This function demonstrates simple nested if-else statements. Only if the condition is false does the execution continue. If, for example, ch is “H”, only the first expression is evaluated. The message “An uppercase letter” is displayed, and the other tests are not performed. Although it is not required, try to put the condition that is most likely to be true as the top condition. If the condition is true, the other tests are not even performed. Such a practice will make your scripts more efficient, especially if the tests are complex. C++ requires a semicolon after the first statement (statement1). Pascal requires that you do not put one there. JavaScript is friendly—it lets you choose.

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Note: JavaScript does not feature a switch statement similar to the one in C++. Here, we’ll refer to the switch statement in C++, and the possible workarounds in JavaScript. When JavaScript supports this statement, it should use the same syntax as in C++. Because JavaScript does not support this statement, you must use nested if-else constructions instead. For example:
if (switchingVariable == value1) statement1 else if (switchingVariable == value2) statement2 else if (switchingVariable == value3)

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statement3 else statement4 // default case

Loop Statements
Loops are control structures that perform a set of actions more than once. Everyone agrees that a computer can calculate faster than a human. Using loops you can repeat calculations and take advantage of the computer’s ability to do them faster. Theoretically, a loop repeats only one statement. However, you already know that a statement can be a block of statements, allowing the repetition of many statements or perhaps the whole program. JavaScript features two basic loop types: n The for loop n The while loop Each loop type has its own advantages. You will learn to choose the most suitable one for your task. for Statement
for ([initialExpression;] [condition;] [operation]) statement

The most commonly used loop is the for loop. Because a loop usually repeats more than one statement, you use a command block in the following format:
for ([initialExpression;] [condition;] [operation]) { statements }

initialExpression is usually a statement or a variable declaration. It should evaluate to a single value and is typically used to initialize a counter variable. This expression may optionally declare new variables with the var keyword. condition is a condition that is evaluated before each successive pass through the loop. The statement is executed only if the condition evaluates to true. operation is a statement that is executed after each consecutive pass through the loop’s body. It is typically used to update or increment the counter variable, which counts the number of passes through the loop. Consider the following looping script fragment:
var number1 = 1 var number2 = 1 for (var counter = 1; counter <= 10; counter++) { document.write(number1 + " ") number2 = number2 + number1 number1 = number2 – number1 }

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This piece of code prints the first ten numbers of the Fibonacci sequence. The Fibonacci sequence is a sequence of numbers that is commonly used in computer science and is intriguing from a mathematical point of view because of certain natural phenomena that exhibit a Fibonacci sequence. The Fibonacci sequence includes the following numbers: 1 1 2 3 5 8 13 21 34 55 89 144 . . . (f1 = 1, f2=1, fx = fx–1 + fx–2) Each number is equal to the sum of the two preceding numbers. The first two numbers are both equal to 1. Take a look at the script. number1 and number2 are initialized to 1, because f1 = 1 and f2=1. The for statement prints the numbers in the Fibonacci sequence. The initial expression of the loop is var counter = 1, which declares the loop counter and initializes its value to 1. The condition counter <= 10 means that the loop executes while the value of counter is less than or equal to 10. Note that the condition is evaluated only at the end of the loop. Incrementing the value of the loop counter is the action in this example, and is typical of for loops. In short, this loop’s attributes cause it to execute exactly ten times. The net effect is that it computes and prints the first ten numbers of the Fibonacci sequence. Understanding the content of the loop is just plain math. After the current number is printed, number2 is assigned the sum of its two preceding numbers in the sequence. number1 should hold the value of number2 before the assignment statement. The following explains how this is done (number2' is the value of number2 before the assignment statement): number1 = number2 – number1 == (number2' + number1) – number1 == number2' Although you do not need to associate a command block with a loop if its body consists of only one statement, it is a common practice in this book. The counter variable is a regular variable. Its value can change during the execution of the loop. You can create infinite executing loops with the for statement. The break statement, which is discussed later, makes such loops meaningful by providing a way to terminate their execution. The basic structure of an infinite loop is:
for ( ; ; ) statement

As you can see, the variables in the preceding example do not have meaningful names. It is common to use such identifiers as loop counters, and most experienced programmers actually do so. Unless the loop counter has a special meaning, name the variable i, j, k, etc. while Statement
while (condition) statement

The while loop continues to loop while the condition (condition) is true. As opposed to the for loop, the while loop is not executed at all if the condition is false at the beginning. That is, the condition is evaluated before each iteration. statement can also be a command block, allowing multiple statements. The following loop calculates n! (n factorial):
var n = parseInt(prompt("Enter a number:", 10)) var nFactorial = 1 // will contain the value of n! var factor = 1 // the current term by which we must multiply while (factor <= n) { factor++ nFactorial *= factor } alert(n + "! is " + nFactorial)

This script asks the user to enter a number. For this example, we assume the user enters a valid number (an integer between 1 and 12). nFactorial always holds the product of integers from 1 (or 2) to factor. Therefore, the loop is terminated after nFactorial is multiplied by factor, when factor is equal to the original number.

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It is possible to create nested while loops. For example, you could change the script that printed a rectangle of asterisks so it uses while loops instead of for loops. You would need to initialize the counter variables before the loop statements, and increment it in the loops’ bodies. You can also use a while statement to create an infinite loop in the following fashion:
while (1) statement

or
while (true) statement

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Tip:

Unlike most C++ compilers, JavaScript’s interpreter does not insist that you use braces even when there is one statement. However, this makes the code more readable so it is a good practice to use them.

Although JavaScript does not feature an until loop, it is very simple to create one with a while statement:
while (!condition) statement

for…in Statement
for (variable in object) statement

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So far I have shown you general loops typical to all programming languages. The for…in loop is quite different. It loops through the existing properties of a given object repeats a variable over all of the object’s properties. variable is the variable that you repeat over the properties. With each pass through the loop, variable is set to the current property. object is the object to which the statement refers. The looping variable is only set to properties of the specified object. Sound a bit complicated? Take a look at the following script segment:
var result = "" // initialize output string for (curProperty in document) { result += "document." + curProperty + "<BR>" } document.write(result)

The order in which they are printed depends on the way they are internally stored. The following function accepts an object and its name. It then iterates over the object’s properties and returns an HTML-ready string that lists the property names and their values.
function getProps(obj, objName) { var result = "" // initialize output string for (var i in obj) { result += objName + "." + i + " = " + obj[i] + "<BR>" } result += "<HR>" // add horizontal rule to output string return(result) // return output string }

The problem with this function is that it cannot print the properties of a nested object. The remedy for this problem—a recursive function—is presented in Chapter 10, Building and Extending Objects. This function is a very helpful tool, specifically for debugging scripts dealing with objects. The following statement shows how to invoke the getProps() function to list the properties of the built-in Math object.
document.write(getProps(Math, "Math"))

Because the function does not send direct output to the screen, you must use a document.write() statement to print the returned value. Notice that the arguments handed to the function in this case are similar: Math and "Math". The first argument holds the object reference itself, and the second one holds the name of the object, which is a simple string. Once inside the function, the object is referenced as obj, so there is no way to extract its original name automatically. The getProps() function implements the array notation to access properties. We use this notation because the property’s name is not a literal but rather a data structure holding a string value. The dot syntax can only be used when the name is in the form of a literal. This rule also applies to methods. Consider the following expressions:
var a = "write" document.write("Test 1 <BR>") document.a("Test 2 <BR>") // error document["write"]("Test 3 <BR>") document[a]("Test 4 <BR>")

The second test generates an error, but the rest print the output as we expect, because the dot syntax accepts only literals as its specifications. break Statement Normally, a loop executes until the terminating condition is met, but you might want it to terminate earlier if it meets some other condition. The break statement does just

As you can see, the loop terminates when i is 50, not when i is equal to 100. That is, it terminates before the condition i < 100 evaluated to false. When i is equal to 50, the condition i == 50 returns true, and the break statement executes, causing the loop to terminate. The break statement lets you place additional terminating conditions at various locations in a loop’s command block. Note that the break statement terminates all loop statements. Although goto statements are disallowed in modern programming, it is sometimes difficult to perform certain actions without the help of this statement. Unfortunately, JavaScript does not provide this tool, so you must find workarounds even for sophisticated structures. Consider the following control structure:
for (i = 1; i <= 10; i++) { for (j = 1; j <= 10; j++) { for (k = 1; k <= 10; k++) { for (l = 1; l <= 10; l++) { } } } }

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Suppose you want to terminate the execution of the entire control structure when l, the inner loop’s counter, is equal to 5. The problem is that a break statement only terminates the loop in which you place it, not other loops that govern it. Using a goto statement in C++, you could have leaped directly to the desired spot. However, JavaScript does not support this statement, so you must find a simple workaround. The following script presents a remedy using an additional variable:
var continueLooping = true for (i = 1; i <= 10; i++) { for (j = 1; j <= 10; j++) { for (k = 1; k <= 10; k++) {

For each pass through the loop, continueLooping is evaluated. If it is false, the current loop is terminated. continueLooping is assigned false in the inner loop when the condition l == 5 is true, and then each loop terminates in turn. Note that it is extremely important to place the if (continueLooping == false) break statements immediately after the closing brace of the nested loop, so that the loop terminates when the nested loop terminates and no other statements are executed in between. The break statement in JavaScript is very important because it is used to create alternatives to loop statements that do not exist in JavaScript. An important loop type in C++ and Perl is do…while. This loop executes a statement as long as the condition is true. Unlike the regular while loop, the condition is evaluated at the end of the loop, so the loop executes once without having tested prior to that. However, JavaScript does not feature such a loop. Once again, it will probably be implemented in a future release of the language. For now, you must come up with a simple alternative using the existing statements. Take a look at the following structure:
while (1) { statements if (condition) break }

Notice that the preceding script segment is equivalent to the repeat…until structure in Pascal, and the do…until in Perl, but it doesn’t match the do…while loop in C++ and Perl. You just have to negate the condition in the following form:
while (1) { statements if (!condition) break }

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continue Statement The continue keyword stops the current iteration of a loop, and continues the execution at the top of the loop. The loop continues to execute until the terminating condition is achieved. The “top” of the loop is: n The update expression in for loops, such as ++i n The condition expression in while loops Here is a simple example for the usage of the continue statement:
var sum = 0 // will hold for (var i = 1; i <= 10; { if (i % 2 == 0) // continue // sum += i // 3: add } the sum of the odd numbers ++i) 1: if i is even 2: go to the top of the loop i to sum (i is always odd!)

This example adds up every odd integer from 1 to 10. The easiest way to do that is to increase the counter variable by 2 after each pass through the loop, but we wanted to feature the continue statement. The loop in the preceding script segment is quite simple. It executes while the value of i is less than or equal to 10. Therefore, the loop executes 10 times, once for each integer value of i. The first line checks if i is an even number, using a simple modulo operation. If the number is even (that is, i % 2 evaluates to 0), the expression i % 2 == 0 evaluates to true. In that case, the continue statement on the second line is executed, causing the loop to start from the top, at the update expression. Here is another example using the continue statement:
for (i = 1; i <= 10; ++i) { if (i == 5) continue document.write(i + " ") }

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The output is:
1 2 3 4 6 7 8 9 10

The 5 is missing because when i is equal to 5 the loop is sent to the beginning, incrementing i to 6. The continue statement is located before the document.write() statement, so the document.write() method is never invoked when i is 5. with Statement
with (object) statement

Writing formal addresses to access properties or invoke methods of the same object is often annoying. The with statement establishes a default object for a set of statements. Within the specified set of statements, any property references that do not

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specify an object are assumed to be governed by the default object defined by the with statement. The same applies to methods belonging to that object. The with statement is useful only when referring to several properties and methods of the same object, so it should always be followed by a command block:
with (object) { statements }

We haven’t discussed user-defined objects, so take a look at an example where the default object is a built-in one:
with (document) { write("Notice that the document object is default") write("<BR>") write("The current background color is " + bgColor) // bgColor is normally document.bgColor }

The output is:
Notice that the document object is default The current background color is #c0c0c0

You will find this statement useful after learning about built-in and user-defined objects. For now, just remember what it does, and use it with your existing vocabulary if you wish. You may recall from the beginning of the chapter that the default object throughout the script is the window object. You can invoke its methods anywhere without literally specifying that they belong to the current window object. For example:
alert("STOP") confirm("OK to proceed?") prompt("How many cookies would you like?", 12)

An alternative syntax specifies the window object, as in window.alert("STOP"). Generally speaking, you can refer to any script as if it were placed in a with statement’s command block, where the argument specifying the default object is window.
<SCRIPT LANGUAGE="JavaScript"> <!-- hide code with (window) { } // end hiding code --> </SCRIPT>

You can also implement nested with statements. In such cases, several default objects exist simultaneously. The scope (where each one exists) is very simple. Inside a command block associated with a with statement, you can refer to the properties and

Summary
This chapter has been rather lengthy but it has established the basic framework of JavaScript for you. It is very important that you are comfortable with all the concepts presented in this chapter before you attempt to continue with the next chapters. This chapter and the preceding three chapters create the foundation for your continued journey into JavaScript.

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Chapter 5

Object-Oriented Programming
Objects
An object is a programming abstraction that groups data with the code that operates on it. All programs contain data of different types. Variables, as well as functions, were introduced separately in the previous chapter. I defined functions, and I defined variables, but I did not connect them in any way. An object encapsulates related data and functions into a single cohesive unit. Originally in procedural programming (Pascal, C, Basic, etc.), code and data were kept apart. For example, in the C language, units of code are called functions, while units of data are known as structures. Functions and structures are not formally connected in C. A C function can operate on numerous data structures, and more than one function can operate on the same structure. In object-oriented (OO) and object-based programming, code and data are merged into an object, which is a single indivisible “object.” An object is simply data and the functions that work on that data placed together in a single package. Objects typically have both properties that are represented by variables and methods which are essentially functions that are part of the object. Figure 5-1 shows several data structures and related operations; this is the definition of an object. All data is located on the left-hand side of the illustration, and operations are located on the right-hand side.

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Figure 5-1. The structure of an object— properties and methods.

Properties
Any physical object has its own unique characteristics. A car, for example, has a size, a weight, a model, a color, a price, and many other attributes. The full set of attributes distinguishes a car from all other objects. These features are called properties or fields in the OO (object-oriented) vernacular. An object’s property stores data related to the object. Properties are usually named according to their meaning. For instance, a property that holds the color of the car would be named color. It is possible to extend an object by adding new properties to it; this was first supported by Netscape Navigator 3.0. Although it is possible to add new properties to an existing object, it is not always possible to modify the value of a static built-in object. For example, the PI property of the Math object cannot be modified, for obvious reasons. JavaScript supports two different types of objects: n Predefined built-in objects, such as the Math object n User-defined objects, such as the course object illustrated in Figure 5-1

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An object can also be a property of another object, thus creating an object hierarchy. An object hierarchy is essentially an object that has other objects as parts of it. The object models of most applications including Microsoft Office are constructed in a hierarchy.

Figure 5-2. A sample object hierarchy.

This figure illustrates an object hierarchy, similar to a family tree. a is only an object, while d, e, f, g, h, and i are only properties. b and c are both objects and properties, and therefore they are located between the other levels of the hierarchy.

Syntax
An object’s properties hold its data. You can refer to properties using the following syntax:
objectReference.propertyName

objectReference is the name of the object that the property belongs to, or any other valid reference to the object. For example, you can assign an object to a variable and then refer to its properties using this variable, followed by the property specification. propertyName is the name of the property (data field). A dot separates each object from its property. A hierarchical object structure, with objects and properties at different levels, uses the same syntax:
object1.object2Property1.object3Property2.property3

Notice that all the elements in the middle are both objects and properties. Referring again to Figure 5-2, let’s assume that the elements of the structure have the following values:

Another important concept is that a property belongs to an object, and only to one object. That is, the same location in memory cannot be associated with two different objects. A property must always be associated with an object. Therefore, the following statements are legal:
var d = a.b.d document.write(d) var e = a.b.e document.write(e) var f = a.c.f document.write(f) var g = a.c.g document.write(g) var h = a.c.h document.write(h) var i = a.c.i document.write(i) // prints 16 // prints 42 // prints true // prints king // prints 13 // prints 10

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As you can see, a variable may be named exactly like a property of an object. This is possible because properties are not at the same scope as variables. However, a variable cannot have the same name as an object at the same level. For example, the statement var a = a.b.d is not valid (actually it is valid, but the value of a is lost). The main object is at the same scope of the variable, because they are not properties of any other object. As a matter of fact, an object is a variable, and two variables that have the exact same name are associated with the same location in memory. The statement var a = a.b.d converts a to a simple variable containing the value 16, and

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not an object. It is generally a good practice to avoid naming variables by an object’s property or method, at least until you feel comfortable with objects and properties. The output of the statement document.write(a), might be “[object create],” because that is the object’s string equivalent.

Methods
During execution of a JavaScript script, an object may invoke one or more methods to accomplish a task. As you know, objects consist of both data (properties) and functions that handle the data. These functions are called methods. Methods enable an object to perform different actions, mostly on its own properties. Methods, as previously stated, are simply functions that are part of the object. However, since they are part of the object they have access to the properties of the object including private properties that cannot otherwise be accessed. Most advantages of OOP (object-oriented programming) are associated with methods. JavaScript does not completely support external libraries (other than the SRC attribute of the <SCRIPT> tag), so the following features apply mostly to built-in objects: n Because an object encapsulates related data and functions (methods) into a single cohesive unit, it is easy to perform maintenance activities. n Every language paradigm influences its application design. Therefore, scripts developed in JavaScript often make use of objects. Although you can design your scripts in a traditional procedural way, you will find it a great deal easier to use objects and methods. You will notice the difference between the traditional style and the OO style as soon as you start writing complex scripts. JavaScript’s implementation of objects is not as powerful as that of Java, so some OO programming advantages that apply to Java do not apply to JavaScript.

Syntax
A method can exist at different levels of an object’s hierarchy. You can invoke a method using the same syntax you use to reference a property, i.e., the “dot” syntax. Methods are actually functions associated with objects. Therefore, they resemble functions in many aspects. A method is called in the following fashion:
objectReference.methodName([arguments])

objectReference is the name of the object, or any other reference. methodName is the name of the method, and arguments are the arguments that the method accepts. Because a method is a function, the name of the method is always followed by a pair of parentheses. This rule also applies to methods that do not accept arguments. You probably find this syntax familiar. We have been using document.write([expression]) to print HTML expressions to the page. write() is a method belonging to the built-in document object.

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Using the Array Notation
You can refer to properties and methods using the “dot” syntax or an array notation. In array notation, square brackets replace the dots. For example, the following expression refers to a.b.d:
a["b"]["d"]

You can use the array notation for both properties and methods. The general syntax is:
objectReference["propertyName"] objectReference["methodName"]([arguments])

It is important to understand this alternative syntax, because you cannot always use the traditional dot syntax. For example, the first character of a property name cannot be a digit when using the dot syntax. When you create an array using the built-in Array object, you can only refer to the elements of the array via the array notation (e.g., myArray[0], myArray[99]). You must always use double quotes when you refer to a property of an object that is not an array. Here are some examples for using the array notation to reference methods and properties:
document["write"]("hello!") window["alert"]("howdy!") // note: alert() == window.alert() Math["PI"]

Sometimes you can only use the array notation. Suppose the variable str holds the string "write". You can use the following syntax instead of document.write():
document[str]()

However, you cannot use document.str() because that is equivalent to document["str"](). Another situation in which you should use the array notation is when you want to name a property not according to the identifier rules. For example, myObject["*"] is possible only with the array notation. When you use the array notation, the value in the square brackets should be a string, because the content is evaluated.

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Object Based Versus Object Oriented

þ

Note: This explanation is intended mostly for programmers who are familiar with object-oriented environments. The basic terms of object-oriented programming will not be explained, as they are beyond the scope of this book. Only the most basic differences between JavaScript and full object-oriented languages are introduced here. You will discover many other minor differences as you learn the language. If you are new to object structures, you should skip this explanation altogether.

JavaScript is based on a simple object-oriented paradigm. This paradigm is often called object based, as opposed to object oriented. Classes do not exist in JavaScript (all

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objects belong to one “class”), nor do packages (because a package groups classes together). The object hierarchy in JavaScript is a containment hierarchy, not an inheritance hierarchy as in Java and C++. That is, an object does not inherit from other objects, but it can be contained by another object if it is a property of that object. Most object-oriented languages require static resolution of objects at compile time. However, an object-oriented language may require dynamic method bindings because polymorphism allows multiple definitions of methods sharing a common name. Calling such polymorphic methods often cannot be resolved until run time. JavaScript is completely based on dynamic binding. That is, object references are checked at run time. There are many other differences between the object paradigm in JavaScript and the one in full object-oriented languages (such as Java and C++).

Summary
This chapter discussed the basics of object-based programming, based on JavaScript’s object model. Its purpose is to introduce you to general object-oriented and object-based terminology and to a bit of JavaScript syntax. Every language paradigm heavily influences the design of applications in that language. Therefore, scripts written in JavaScript are typically object-based systems. Procedural scripting is possible in JavaScript, but object-based scripting has many advantages. Because JavaScript has a large set of built-in objects, including very useful methods and properties, a basic understanding of the object structure is necessary. If you are a beginner and find objects difficult to understand, don’t be too concerned. I promise that you will understand these concepts as we move on, mostly by studying examples.

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Functions and Variable Scope
Variable Scope and Storage Class
All JavaScript variables have two attributes: n Scope n Storage class JavaScript implements these characteristics slightly differently from other programming languages. The scope of a variable describes the area of the script where the variable is valid. You can refer to the variable only in this area—it does not exist elsewhere. The scope of a global variable is the entire script. You can access it anywhere between the <SCRIPT> and </SCRIPT> tags and other scripts that are executed after the variable definition (including preceding event handler scripts). In JavaScript, you can declare global variables with or without the keyword var. It does not affect the scope of the variable at all. A local variable is declared in a function. It can only be referenced by statements inside that function. Therefore, a global variable may have the same name as a given local variable. In the scope of a local variable (the function where it is defined), only the local variable exists. Outside that function, only the global one exists. A local variable must be declared inside a function using the keyword var. If the keyword is omitted, the variable is assumed to be global. The following script segments show how a variable’s scope influences various results:
// Script 1 function test() { age = 43 } age = 11 test() document.write(age + "<BR>") // Script 2 function test() {

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var age = 43 } age = 11 test() document.write(age + "<BR>")

The only difference between these scripts is the statement inside the function. In the second script segment, we use var to define the variable age, whereas in the first statement we simply state the desired variable name. The output of the first script is:
43

and the output of the second script is:
11

Take another look at the first script and try following the execution thread. The value 11 is assigned to the global variable age and the function is invoked. Notice that the word var is not used inside the function. Therefore, the variable age in the function is assumed global, and the value 43 is assigned to the global age. Thus, the result is 43. In the second script, we assign the value 11 to the variable age, and then call the function. The var keyword means that the variable age, which is defined inside the function, is local. It does not exist beyond the function, so the value of the global variable age (the one defined before the function call) is not modified by the assignment statement inside the function. The last statement of the script prints the value of the global variable age, because it is the only variable named age in that scope. Therefore, the script’s output is 11—the global variable’s initial value. Once declared with the var keyword, additional statements referring to the variable should not issue that prefix. Here is the previous script with an additional statement inside the function:
// Script 2 (additional statement in function) function test() { var age = 43 age = 58 } age = 11 test() document.write(age + "<BR>")

Once again, the output of the script is 11. The variable inside the function is declared with the keyword var. From that point on, the scope of the variable is only the function test(). It does not exist outside the boundaries of that function, so the script’s output is the value of the global variable named age, not the local one. Note that the statement:
age = 11

in the previous script segments is equivalent to:

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var age = 11

because the var keyword is optional for global variables. Here’s another script segment that deals with various scopes:
function test() { age = 5 } test() document.write(age + "<BR>")

The script’s output is 5, because the variable age is declared inside the function as a global one (without var), so its scope is the entire script. The next script generates quite an unexpected output:
function test() { var age = 5 } test() document.write(age + "<BR>")

Rather than generating an output, this script generates an error because age is a local variable and the printing statement is not located in its scope. The variable’s storage class may be either permanent or temporary. You cannot modify this attribute directly, the way you can declare the variable as static in other languages such as C++. The storage class of a variable in JavaScript depends only on its scope. Local variables are temporary, while global variables are permanent. Permanent variables exist throughout the script and even in other scripts. Even after a script has terminated, the global variable remains, and is only discarded when the page unloads. Temporary variables are allocated on the stack (a section of memory) when a function is called. The space used by a function’s local (temporary) variable is returned to the stack at the end of the function execution. After a local variable “dies,” it leaves free memory space for other variables and objects to come. Avoid declaring large local data structures such as arrays (discussed in the following chapters). If you try to allocate too many temporary variables or extremely large ones, you are likely to receive an error (usually called stack overflow). For the same reason, avoid passing large data structures to a function. It is better to use global variables for such tasks. Function parameters are identical to local variables. They exist only inside the function, and do not affect any variable outside it. You can refer to function parameters as if they were variables that you declared at the beginning of the function and initialized with appropriate values. The following script segment demonstrates this:
function test(age) { age = 5 document.write(age + "<BR>")

(g) stands for global, (functionName) stands for local in the function named functionName.

This technique is very useful because JavaScript does not have a decent debugger. Make sure you understand every value in the table. If you don’t have much programming background, Table 6-1 might seem confusing. However, all you must do to use the table is locate the line where the printing statements are placed and extract the variables’ values from the table. For example, suppose you want to find what the statement prints on Line 13. The last line expressed in the table before Line 13 is Line 12. We are trying to find out what the value of apple is, so you should first check to see if there is a local variable with the corresponding name, because a local variable rules if it exists. As you can see, the value of the local variable apple is 2. Therefore, JavaScript prints that value on Line 13, followed by a line break. The following script’s output might surprise you a little:
var num = 4 function funct1() { alert(num) var num = 2 } funct1()

The output of this script is illustrated in Figure 6-1:

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Figure 6-1. The “undefined” alert box.

The term “undefined” means that a variable does not have a value at the desired point, although it has previously been declared. Generally speaking, JavaScript scans the entire script before any execution begins. During this scan, also called parsing, JavaScript evaluates each line of code, testing it for basic syntax errors. It also determines the scope of each variable, regardless of whether it is in a function or not. If it encounters a variable declaration with the keyword var, that variable is considered local, even if a global variable with the same name exists. A variable’s scope can be either the entire script or just one function. In the preceding script segment, a global variable num is declared by initializing it to 4. The function consists of one local variable declaration. Note that the variable num inside the function has no connection to the global variable num. The function is executed, as usual, from top to bottom. The first statement instructs JavaScript to display the value of num, the local variable inside the function. JavaScript already knows that the local variable is declared inside the function, so it does not generate an error. However, at that point, the variable does not retain any value, so it is undefined. The following definitions of a function are equivalent:
// #1 function functionName { [JavaScript statements...] var varName = initialValue [JavaScript statements...] } // #2 function functionName { var varName [JavaScript statements...] varName = initialValue [JavaScript statements...] }

Some programmers find it easier to use the second form. Each form has its own advantages and disadvantages—choose the one that is most convenient for you. In this book, we use the first form, because we prefer to use the var keyword in an assignment statement. If you declare global variables inside a function, you must assign an initial value to the variable; otherwise, JavaScript returns an error. The following script segment and screen capture demonstrate this fact:

Figure 6-2. The error message returned when a global variable is declared without an initial value.

Notice that the variable does not hold an undefined value as it would if the declaration used the var keyword—it causes an error instead. The var keyword is used to “officially” declare variables. JavaScript remembers only the scope of variables that are declared in that fashion. It recognizes other variables during the execution of the script, and automatically refers to them as global variables. Although it does not really matter whether or not you use var to declare global variables in the main script, it might affect the result of the script under certain circumstances. If you do not use var and, when the variable is still undefined, you use it in a value-requiring statement, an error will occur. But this is an easy problem to fix because you will get a message. The problem with the var declaration is that not all statements that require a variable with a meaningful value use the undefined string when the variable has no value. The basic document.write() statement may print nothing at all if the variable is undefined.

What are Functions?

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Note: Throughout this chapter and the previous ones, we did not pay much attention to many important details related to functions. The remainder of this chapter discusses parameters, return values, and other important concepts. You should read this material, even if you feel comfortable with functions. A complete understanding of functions will be necessary later when we discuss objects and methods.

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with related data. JavaScript features many built-in functions which are presented later in the book. Such functions are predefined.

Defining and Calling Functions
We have spoken much about defining functions. The general syntax of a function definition is:
function functionName([parameters]) { statements }

and the form of a function call is:
functionName(arguments)

Function Parameters
Sometimes you may want to create a function that accepts values when it is invoked. The function should then be able to use these values to perform a specific task. When you define a function, you should specify the names by which you refer to the custom values handed off to the function from outside. These names must follow the same rules that apply to identifiers in the language. Parameters act like local variables, so they exist only inside the function where they are defined. Therefore, a parameter may use the same name as a global variable or a local variable in a different function. You can manipulate and modify the value of a parameter as if it were a common variable. There is no need to explicitly declare a parameter inside the function’s body as you would with a regular variable. JavaScript is loosely typed, so you do not specify the data type of the arguments as in C++, Java, Pascal, and other strictly typed programming languages. Both variables and literals can be passed to a function. All parameters in a function definition header should be delimited by the comma operator (,). Here is a simple JavaScript function with two parameters:
function printName(name, ruleWidth) { document.write("<CENTER><H1>" + name + "</H1></CENTER>") document.write("<HR WIDTH=" + ruleWidth + "%>") }

You can call this function with a simple function call, such as:
printName(prompt("Enter your name:", "John Doe"), 60)

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Note: Arguments are the specific values you assign to parameters. These terms are sometimes considered exchangeable, even by advanced programmers. It is sometimes difficult to distinguish between them, so you might disagree with some of the terms in this chapter.

Using the Argument Array
JavaScript supports functions that accept a variable number of arguments. The first argument is functionName.arguments[0], the second one is functionName.arguments[1], the third one is functionName.arguments[2], and so on. The number of arguments handed to the function is stored in the length property of the arguments object: functionName.arguments.length The following script demonstrates this concept:
function calc() { document.write("The first argument is ", calc.arguments[0], "<BR>") document.write("The fourth argument is ", calc.arguments[3], "<BR>") document.write("There are ", calc.arguments.length, " arguments<BR>") } var company = "Yahoo" calc(2, 999, "internet", company, 0.0)

The script’s output is:
The first argument is 2 The fourth argument is Yahoo There are 5 arguments

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Caution: Netscape Navigator allows you to use arguments rather than functionName.arguments. However, Microsoft’s Internet Explorer 3.0 generates an error when you implement that shortcut. If you want to be on the safe side, you can use the following statement at the beginning of the function:
var args = functionName.arguments

You can then use args in the regular way (args[0], args.length, etc.). The ability to refer to the arguments passed to the function during the last call of the function often trips up JavaScript scripters. For example, consider the following script:
function func() { document.write(func.arguments[0] + "<BR>") }

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func("a") func("b") func() func("c")

The output of this script is:
a b b (!) c

The only surprising line is the third one. The argument printed is the one passed to the function during the previous call. The first argument of the current call cannot be printed because there is no such argument. In Internet Explorer this will print as “undefined”. At times, you will receive unexpected results from the func.arguments.length property, as well as from the elements of the array itself. Note, also, that you may only refer to the arguments array from within the function.

The scope of the arguments object’s properties is the current function, so it can only be used inside a function. You can use loop statements to print a list of arguments handed to a function:
function createList() { var result = "" for (var i = 0; i < createList.arguments.length; ++i) { result += createList.arguments[i] + "\r" } alert(result) }

Here is a simple function call:
createList("C", "H", "U", "C", "K”)

When invoked with the preceding statement, the function generates the following dialog box:

Figure 6-3. Each line displayed is a distinct argument handed to the createList() function.

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Creating Functions with Default Arguments
JavaScript does not support functions with default arguments. In C++, these are the values supplied to the parameters in a function’s prototype. Calling the following function at the beginning of a function is a simple workaround you can use in JavaScript:
function checkDefault(parameter, defaultValue) { if (typeof parameter == "undefined") return defaultValue /* else */ return parameter }

The parameter is assigned its default value only if the function is called without specifying the required argument. The following function uses this technique to print a row of a given character; an asterisk is the default character, and 30 characters is the default length:
function checkDefault(parameter, defaultValue) { if (typeof parameter == "undefined") return defaultValue /* else */ return parameter } function drawLine(character, numOfChars) { character = checkDefault(character, "*") numOfChars = checkDefault(numOfChars, 30) for (var i = 0; i < numOfChars; i++) { document.write(character) } }

It is also possible to specify the second argument without providing the first, using an undefined variable (just declare it before with var, without initializing it).

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Caution: Be aware that the preceding checkDefault() function is not compatible with Netscape Navigator 2.0x or Microsoft Internet Explorer 3.0x, because they do not support the typeof operator. However, this is not a problem with the most recent versions of these two browsers. Use the arguments array to simulate C++’s default arguments feature. The previous drawLine() function would then be as follows:
function drawLine(character, numOfChars) { args = drawLine.arguments if (args.length < 1) character = "*" if (args.length < 2) numOfChars = 30 for (var i = 0; i < numOfChars; i++) { document.write(character) } }

If the function receives no arguments, the value of character is set to the default “*”. If the function received one argument or less, the parameter numOfChars defaults to 30. Because this function uses the arguments array, named parameters are not necessary—you can use the array instead as in the following function definition:
function drawLine() { args = drawLine.arguments if (args.length < 1) args[0] = "*" if (args.length < 2) args[1] = 30 for (var i = 0; i < args[1]; i++) { document.write(args[0]) } }

Returning a Value
A function can return a value, so it is possible to use a function call as a valid JavaScript value. Use this feature to create general functions for usage by other scripts. For example, you can use a function that accepts a number in one base and returns it in a different representation. This function can be used in creating a base-conversion table, a dialog box, or additional calculations. Use the following syntax to return a value from within a function:
return value

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The return statement returns a value and terminates the function, similar to the way break terminates a loop. Take a look at the following function:
function sum(op1, op2) { return op1 + op2 } var answer = sum(3, 5) alert(answer)

The value returned by the function is the sum of its arguments. As you can see, the function evaluates to a value. A function may only return one value—either a variable, a literal, or any other valid data structure. A function with a return value can also generate an output. When you use the function in an expression, it automatically runs, executing all statements until it returns a value that is used in the expression. The preceding function can also display the sum of its arguments inside the function itself, provided that the alert() statement precedes the return statement (which terminates the function):
function sum(op1, op2) { var result = op1 + op2 alert(result) return result } var answer = sum(3, 5)

Be sure that the return statement is placed after the statements that should be executed, because it terminates the function and no additional statements of the function are executed afterwards. You can use nested if constructions without else statements, provided that each if statement calls a return statement if the condition is true. Eliminating the else statements does not affect the efficiency of the function, because the function terminates immediately after a true condition is met. The following function converts hexadecimal digits (not numbers) to their equivalent decimal values:
function getValue(dig) { if (dig == "A") return 10 if (dig == "B") return 11 if (dig == "C") return 12 if (dig == "D") return 13 if (dig == "E") return 14 if (dig == "F") return 15 return dig // the return statement terminates the function, // so there is no need for else statements }

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If the digit is between 0 and 9, none of the conditions evaluate to true, so the function returns the same digit it accepts:
return dig

Recursion
A recursive function is usually one that calls itself. It is similar to a loop with or without a terminating condition. Some programming functions lend themselves to recursive algorithms, such as the factorial one. The factorial is a classic recursive algorithm, because it is very obvious. A simple mathematical definition of the factorial operation is:
fact(0) = 1 fact(n) = n * fact(n–1)

As you can see, the algorithm works its way down from the given number to zero, so when it gets to fact(0) it must work its way back up to the numbers. We know that fact(0) = 1.
fact(1) = 1 * fact(0) = 1 * 1 = 1 fact(2) = 2 * fact(1) = 2 * 1 = 2 fact(3) = 3 * fact(2) = 3 * 2 = 6

You can call the function in the following way:
var fiveFactorial = fact(5) // 120

Notice that the else statement is commented because it is not necessary. The fact() function satisfies two rules. First, it has an ending point. Second, it simplifies the problem because fact(num – 1) is simpler than fact(num). Recursive functions should always follow these two rules.

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Recursive functions have a few advantages: n Invariably, recursive functions are clearer, simpler, shorter, and easier to understand than their nonrecursive counterparts. n The program directly reflects the abstract solution strategy (algorithm). The recursive factorial function, for example, uses the common mathematical strategy to solve the problem. At first, it may seem that recursive functions are more difficult, but they become easier with practice. Recursion can also be used to compute a value in the Fibonacci sequence (1, 1, 2, 3, 5, 8, 13, ...). The basic algorithm is:
getVal(1) = 1 getVal(2) = 1 getVal(place) = getVal(place – 1) + getVal(place – 2)

Note: The bottom row specifies the place; the top row specifies the Fibonacci value at that space.

Notice that we once again left out the unnecessary else statement. Some programmers prefer not to use complex expressions and values returned by other functions with the return statement. Instead, they assign those values to variables and then return them:
function getVal(place) { if (place == 1 || place == 2) return 1 var s1 = getVal(place – 2) var s2 = getVal(place – 1) return s1 + s2 }

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Tracing Values in Recursive Functions
Sophisticated recursion algorithms are often difficult to follow. Many values change, some in the way you expect, while others make surprising “moves.” JavaScript still does not have a decent debugger to help us with that, so we must develop our own tools, using output statements to evaluate expressions and variables. With a few additional statements, it is possible to display the values of the variables and parameters throughout the recursive execution. Here is the getVal() function with a few additions to track values during the recursion:
text = "place\t\ts1\t\ts2\t\tval\r" text += "===================================================\r" function getVal(place) { if (place == 1 || place == 2) return 1 var s1 = getVal(place – 2) var s2 = getVal(place – 1) text += place + "\t || \t" + s1 + "\t || \t" text += s2 + "\t || \t" + (s1 + s2) + "\r" return s1 + s2 } getVal(6) alert(text)

Note that it is legal to not use the value returned by a function (getVal(6) in this case). In the preceding script segment a global variable (text) is used to store the formatted tracing of the execution course. A self-formatted table is created and displayed in an alert box after the execution terminates. The following values are put in each row of the table: n The value of the parameter (place) in the current function call. The assignment to text is done at the end of the function (after the recursive calls to the function), so the first function in the table is the last to be called. n The values of s1, s2, and their sum The exact output of the preceding script segment (the argument handed over to the function is 6) is:

Figure 6-4. A formatted table that helps a scripter trace variables through a recursive function.

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From here it is effortless to track all the function calls and the values of the variables during the recursion. Use recursive algorithms carefully. Do not create functions that call themselves hundreds of times if there is a simple non-recursive alternative. If you doubt this caution, try calculating the 100th value of the Fibonacci sequence!

Variables and Recursive Functions
You probably noticed the use of local variables in the previous Fibonacci recursive function. You should regularly use local variables in recursive functions. Make this a habit so you never forget, because it is laborious to detect variable scope-related problems, even when you know the function does not work correctly. This is where variable tracing tables come in handy. Here is the preceding script with the keyword var absent:
text = "place\t\ts1\t\ts2\t\tval\r" text += "===================================================\r" function getVal(place) { if (place == 1 || place == 2) return 1 s1 = getVal(place – 2) s2 = getVal(place – 1) text += place + "\t || \t" + s1 + "\t || \t" text += s2 + "\t || \t" + (s1 + s2) + "\r" return s1 + s2 } getVal(6) alert(text)

6
An obvious error appears on the sixth line, when the function is called with the value 5. The function returns 4 instead of 5. The seventh line (where place is 6) also reflects an error. The function should return 8, not 5. The first mistake encountered appears to be when the function is invoked with the argument 5. The problem there is the variable s1, because it holds an incorrect value. The recursive algorithm seems to Chapter

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be fine, so the only possible mistake is with the variables. The exact problem is that the variables are global, not local. Focus on the call to the function, with the value 5. That function then calls itself with the value 3, and after a few more recursive calls, the correct value is returned to s1. Remember that this variable is global. The first function execution (with 5) now calls itself again, this time with the value 4. A few more recursive calls take place, during which the values of s1 and s2 are modified. s1, like s2, is global, so its value is not the same value that was assigned to it in the initial execution of the function (with the argument 5). It holds a value assigned to it by a deeper function call, which is obviously incorrect for the current function. The wrong value of s1 at this point influences the result. The same problem applies to all function calls with a value greater than 4. If you do not understand the cause of the problem, use the following script:
var text = "" function getVal(place) { if (place == 1 || place == 2) return 1 s1 = getVal(place – 2) var tempS1 = s1 // assign correct s1 value to tempS1 s2 = getVal(place – 1) // check if s1 was modified by the preceding statement if (tempS1 != s1) { text += ">>>\tThere appears to be an error in the " text += "current execution of the function (place = " text += place + ") —\rthe value of s1 has unexpectedly" text += " been modified (" + tempS1 + " => " + s1 + ")\r" } return s1 + s2 } getVal(6) alert(text)

Summary
In this chapter we focused on advanced function techniques and variable scope. Failing to pay attention to the variable scope concept results in hours of frustrating work trying to find bugs in simple scripts. After attaining a deeper understanding of variables and their scope, you will find it easy to write scripts with many functions, interleaving local and global variables. This chapter also presented some important matters related to parameters, such as referring to a variable number of arguments. Scripting specially designed dialog boxes took advantage of this feature. Another important concept discussed in the chapter is recursion. We found out how to debug recursive algorithms with a formatted table that traces variables.

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Utilizing JavaScript Operators
Operator Categories
JavaScript has many operators, most of them borrowed from C and Perl. The wide variety of operators makes it necessary to divide them into the following categories: n Mathematical operators n String operators n Bitwise operators n Assignment operators n Relational operators n Short-circuit logical operators n More logical operators n Data type operator n Void operator In this chapter we will discuss JavaScript’s operators, grouped by the above categories. Operators in each category are divided into two groups: n Unary operators—operators that operate on a single operand n Binary operators—operators that operate on two operands

The division operator also performs the usual arithmetic conversion. However, since JavaScript is loosely typed, this operator does not act exactly as in C, Perl, and other strictly typed programming languages. In those languages, integer division is different

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from floating-point division in that the result of integer division is always an integer number. JavaScript, on the other hand, does not explicitly distinguish between integers and real-valued numbers, and therefore, the result of a division operation is not guaranteed to be an integer number. In fact, most floating-point numbers are the result of a division operator. While debugging a script, it may be helpful to remember that the division operation in JavaScript generates the same value as your pocket calculator. You should also remember that the remainder of a division operation is never discarded. When the operands are floating-point numbers and cannot be represented in binary notation, division expressions often evaluate to inaccurate results. The following demonstrates the behavior of JavaScript’s division operator:
3 / 4 // evaluates to 0.75 3.6 / 0.1 // evaluates to 36 –20 / 4 // evaluates to –5 11.1 / 2.22 // evaluates to 4.999999999999999

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Modulus
operand1 % operand2

The modulus operator returns the remainder of a division operation. The division is performed, but only the remainder is kept. The sign of the result is the sign of the quotient. The modulus operator in JavaScript is also different from the one in other programming languages. It operates not only on integers but also on floating-point numbers. You should be aware that the modulus operator occasionally returns inaccurate results. The modulus’ inaccuracies stem from the division operation which sometimes returns inaccurate results:
12 % 5 // evaluates to 2 12.3 % 4 // evaluates to 0.3000000000000007 (inaccuracy) 0 % 99 // evaluates to 0 12.75 % 4.25 // evaluates to 0 11.1 % 2.22 // evaluates to 2.219999999999999

The Nonexistent Integral Division Operator
JavaScript does not feature an integral division (also called div) operator. However, it is easy to create such an operator using simple arithmetic operators. The following function demonstrates this capability:
function div(op1, op2) { return (op1 / op2 – op1 % op2 / op2) {

The keyword return instructs the function to return a value, so the function call itself evaluates to a value, just like an expression consisting of an operator. Now we can define the newly created div syntax:
div(operand1, operand2)

The function evaluates to the quotient of its arguments, with the remainder discarded. The sign of the result is the sign of the quotient.

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Caution: Various operations on floating-point numbers return inaccurate results. This may seem strange, but such inaccuracies are a common phenomenon in computers. You should avoid floating-point values when possible, because such inaccuracies often result in unexpected behavior. Above all, debugging scripts that fail to work due to inaccurate calculations is nearly impossible.

Increment
operand1++ ++operand1

The increment operator is a unary operator, which can be used in either suffix or prefix notations. It increments the operand’s value by 1. If used after the operand (suffix), the operator returns the value of the operand before incrementing it. If used before the operand (prefix), the operator returns the value of the operand after incrementing it. Understanding these differences is important when you use such operations as side effects of other statements, such as assignment statements. The following set of statements outlines this concept:
var a = 1 var b = ++a // prefix document.write("a is ", a, ", b is ", b) // a is 2, b is 2

The first statement assigns the value 1 to a. The second statement performs two different actions: n Increments a to 2 n Assigns a’s new value to b The increment operator in suffix notation performs the actions in reverse order, and therefore the results differ. Suffix notation is demonstrated in the following code:
var a = 1 var b = a++ // suffix document.write("a is ", a, ", b is ", b) // a is 2, b is 1

b is assigned the value of a, and then a is incremented. Dual actions in one statement are discussed at the end of this chapter. Generally, you should avoid using such side effects. The previous code would be simpler had it looked like:

The increment operator can only be used with a variable or a property of an existing object, but not on a literal. It is natural to come to a conclusion that incrementing is the same as adding 1 to the value:
var a = 1 var b = 1 a++ b = b + 1 // equivalent to b += 1 (see assignment operators)

This is true as far as correctness of the script is concerned. It is incorrect if performance is important. The advantage of incrementing is that it is much faster than standard assignment (fourth line in above code section). You should always increment when you want to add 1 to a variable (or to a property of an object). It is not so important when the addition operation is done a few times. You will definitely feel the difference when you have 100,000 addition operations. Another benefit of the incrementing operator is that it is much easier to understand a statement like countTemp++ than countTemp = countTemp + 1. It is important to remember that Boolean expressions are equivalent to 1 and 0 in certain situations. The following statements show the effect of incrementing Boolean variables:
var a = true var b = false a++ b++ document.write("a is ", a, ", b is ", b) // a is 2, b is 1

Decrement
operand1---operand1

The decrement operator is similar to the increment operator. It decreases the value of the operand by 1, whereas the increment operator increases it by 1.

Negation
–operand1

The negation operator precedes a numeric value (a variable, a property of an existing object, or a numeric literal). By placing this operator before its operand (do not insert any space characters), JavaScript evaluates a positive number as its corresponding negative number and vice versa. As before, you might think that this operator can be replaced by a statement in which the operand is multiplied by –1. Once again, this is a

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mistake. Due to the internal structure of the JavaScript interpreter, and the negation operator specifically, negating a numeric value using the negation operator is faster than multiplying it by –1. If you are a traditional Pascal programmer, it might take you a while to get used to the negation and incremental operators, but it is worth the effort!
var a = 3 var b = 9 –a + b // evaluates to 6 –b // evaluates to –9

String Operators
operand1 + operand2

The string operator’s syntax is identical to that of the addition operator. They differ in the type of operands they operate on. This operator accepts any values as operands, provided that at least one of them is a string. A string is actually an object, so it can be said that the string operator operates on string objects. It joins them together, as in:
"Ladies " + "and " + "gentlemen"

The string operator can operate on more than two operands, but it is still a binary operator, because of the way it works. It concatenates the first two strings, then concatenates the third string to the accumulated string, and so on. If one of the operands is not a string, it is automatically cast to a string. The string operator is also called a concatenation operator. An expression consisting of numerous string operators evaluates to a single string. Based on that, here are two different statements:
document.write("I have " + 2 + " cookies.") document.write("I have ", 2, " cookies.")

At first, you might think that these statements are equivalent. They aren’t, because the first one uses the string operator, and the second one uses commas to delimit strings and numbers. They differ more than in style. In the first statement, the expression between the parentheses is evaluated to a single string—"I have 2 cookies." Therefore, the document.write() method in this statement prints only one expression. The second statement prints multiple expressions. The literals are not evaluated to a single value as in the first statement but are rather printed independently. Both statements print the same HTML to the page, but they do it in different ways. In order to understand how each statement works, take a look at the following sequences of statements. The first sequence is equivalent to the first statement in the previous set, and the second sequence is equivalent to the second statement.
// sequence #1 var stringToPrint = "I have " + 2 + " cookies." document.write(stringToPrint)

A common mistake made by beginners is to forget spaces in strings. A space is a character just like any other. Forgetting a space character is not a severe error, because you can easily locate where to add it. You should use one of the following statements to print two consecutive numbers with a separating space character in between:
document.write(16 + " " + 18) // first possibility document.write(16, " ", 18) // second possibility

The first statement is valid because of the automatic casting method used by JavaScript’s interpreter.

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The plus (+) operator in JavaScript does not convert string operands to numeric values. The reason behind this is that the string concatenation operator and the plus operator use the same character: +. If only one operand is a string, the other is converted to a string value and then the operator concatenates the strings. In Perl, the plus (addition) and minus (subtraction) operators convert their arguments from strings to numeric values if necessary, and return a numeric result. This feature is made possible because the string concatenation operator in Perl is . (dot), not +, which is shared with another operator (addition) in JavaScript. C++ and Pascal feature a function that concatenates strings.

Bitwise Operators
First, let me state that bitwise operations can be confusing for many programmers. I should also note that a great many programmers work for years without ever having to use them. You can be a successful JavaScript programmer even if this section does not quite make sense to you. However, I would have been remiss not to have included it. Bitwise operators are the operators used in bit-oriented operations. A bit is the smallest unit of information, usually represented by 0 or 1. Bit manipulations are used to control the machine at the lowest level. If you plan to program at a higher level, this section may be safely skipped. In JavaScript you won’t be using bitwise operators to control the machine at a low level but rather for other purposes such as encrypting and encoding. Eight consecutive bits form a byte. There are 256 (28) byte variations. That is, a byte can be one of 256 eight-bit sequences. For example, 11010001 is one of these 256 possibilities. A byte is represented by a character in programming languages that support character data types, such as C, C++, and Pascal.

The bitwise AND operator compares two bits. The only situation in which the result is 1 is when both bits are 1. Here is the truth table for this operator:
Table 7-4. Bitwise AND truth table. Bit1 0 0 1 1 Bit2 0 1 0 1 Bit1 & Bit2 0 0 0 1

The AND operator, like all other bitwise operators, can take only a numeric value as its operand.

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Let’s take a look at the following numbers, and how the bitwise AND operates on them:
0x23 & = 0x72 0x22 001000112 011100102 001000102

0x23 & 0x72 // evaluates to 34 (= 0x22 = 2216)

The bitwise AND operator is similar to the logical AND operator, which is discussed later in this chapter. You can use the bitwise AND operator to test whether a number is even or odd. In binary (base 2), the last digit of an odd number is 1, and the last digit of an even number is 0. The following function uses the bitwise AND operator to determine whether the number is odd or even. It returns true if decimalNumber is even, and false if it is odd.
function checkEven(decimalNumber) { return (decimalNumber & 1 == 0) }

Don’t worry if you are not familiar with the “==” equality operator. It is introduced later in this chapter. Come back to this script after we discuss the equality operator.

Bitwise OR
operand1 | operand2

The OR operator, also known as the inclusive OR operator, compares its operands. It returns 1 if at least one of the compared bits is 1. Table 7-5 shows the operator’s truth table.
Table 7-5. Bitwise OR truth table. Bit1 0 0 1 1 Bit2 0 1 0 1 Bit1 | Bit2 0 1 1 1

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Although you will probably never use bitwise operators, let’s learn how the results are calculated. All calculations will be performed in hexadecimal and binary bases, because they are most convenient for the task. Remember that hexadecimal numbers have a “0x” prefix.

Take a look at the following example:
0x2C ^ = 0xA3 0x8F 001011002 101000112 100011112

The bitwise XOR operator is used in a simple cipher technique called XORing, which will be covered later in this book.

Bitwise NOT
The bitwise NOT operator returns the reverse of its operand. Therefore, it is also called the bit flip operator or the invert operator. All 1s are converted to 0s, and all 0s are converted to 1s. Here is the truth table:
Table 7-7. Bitwise NOT truth table. Bit 0 1 ~Bit 1 0

Note that this operator refers to all operands (integers) as 32 bits. If they are not 32-bit integers, they are converted for the operation and then converted back to their initial form.

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Shift Operators
The shift operators take two operands: n The quantity (value) to be shifted n The number of bit positions to shift the first operand Shift operators convert their operands to 32-bit integers, and return a result of the same type as the left operator. There are three shift operators: n Left shift (<<) n Right shift (>) n Zero-fill right shift (>>) Left Shift
operand1 << operand2

The left shift operator shifts the first operand the specified number of bits to the left. All bits that are shifted out (to the left) are discarded. New bits coming in from the right are zeros. In the following example, represents 4 bytes (32 bits):
179 0xB3 101100112 00000000000000000000000010110011 00000000000000000000000010110011
empty space filled with 0s

179 << 0 179 << 2

00000000000000000000001011001100 10110011002 0x2CC 716

You now know that 179 << 2 is 716. You might realize that 179 * 4 is also 716—this is no coincidence. In general, x << n is the same as x * 2n. Shifting left n places is the same as multiplying by 2n. This rule applies also to negative numbers—the sign is always preserved. Thus, you can choose between two different methods to multiply by a power of two. Although shifting is faster, it is also less clear to the reader and should be avoided. Right Shift
operand1 > operand2

The right shift operator is also called sign-propagating right shift because it preserves the sign of the initial operand. Like the left shift operator, the right shift operator shifts the first operand the specified number of bits to the right. Excess bits shifted off at the right are discarded. The sign of an integer is stored in the first bit (from the

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left). In order to preserve the sign, the first bit remains as is. The other 31 bits are shifted to the right. New bits coming in from the left are either 0s or 1s, depending on the sign. If the first bit was a 0, all new bits are 0s, and vice versa. Therefore, if the number was positive, the operation will also return a positive number; if it was negative, the operation evaluates to a negative number. It is important to remember that this operator, like all shift operators, converts the first operand to a 32-bit integer, and after shifting returns it to its initial representation. Here is a simple example:
176 0xB0 101100002 00000000000000000000000010110000 0 00000000000000000000000010110000 00000000000000000000000000010110 101102 0x16 22

176 > 0 176 > 3

Let’s see how this operator works on a negative number, say, –17.
–17 111111111111111111111111111011112 11111111111111111111111111101111 1 1111111111111111111111111101111 11111111111111111111111111111101 111111111111111111111111111111012 –3

–17 > 0 –17 > 3

You can refer to the right shift operator as the opposite of the left shift operator. While the left shift operator is equivalent to multiplying by a power of 2, the right shift operation is equivalent to dividing by a power of 2. 176 > 3 is equal to 176 / 23. In general, x << n is the same as x / 2n. Zero-fill Right Shift
operand1 >> operand2

The zero-fill right shift operator shifts the first operand the specified number of bits to the right. Like the sign-propagating right shift, the zero-fill right shift discards excess bits that are shifted off to the right. However, the bits shifted in from the left are always zeros. The number’s sign is lost because the leftmost bit is always 0. Here is the example from the previous section, this time with the zero-fill right shift:
–17 111111111111111111111111111011112 Þ 11111111111111111111111111101111 11111111111111111111111111101111 00011111111111111111111111111101 111111111111111111111111111012 536870909

–17 >> 0 –17 >> 3

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Don’t worry if the bitwise shift operators seem difficult.

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Assignment Operators
operand1 ¨ = operand2

Assignment operators are binary operators handling arithmetic, string, or bitwise operators. They perform the regular operation (¨) on the operands and assign the result to the first operand. The assignment operators are as follows:
Table 7-8. Assignment operators. Syntax = += –= *= /= %= &= |= ^= <<= >= >>= Name Equals Add/concatenate by value Subtract by value Multiply by value Divide by value Modulo by value Bitwise AND by value Bitwise OR by value Bitwise XOR Left shift by value Right shift by value Zero-fill right shift by value

You already know what the simple = assignment operator does—it stores the value of the expression in the data structure. For all the other operators, JavaScript pretends that:
var1 ¨ = var2

is:
var1 = var1 ¨ var2

For example:
counter >= 2

shifts the value of counter two positions to the right, and
text += " Gates"

attaches the word Gates to the end of the string stored in text. The same rule applies to all assignment operators listed in Table 7-8. Since the assignment statements are evaluated from right to left, you can use multiple operators in the same statement. The rule is that the value to the right of the operator is evaluated and then assigned to the variable to the left of the operator.

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num1 = num2 = num3 = num4 = num5

The value of num5 is assigned to num4, the value of num4 is assigned to num3, and so on. After such an operation, all five variables hold the same value.

Relational Operators
operand1 _ operand2

Relational operators, also called comparison operators, compare two values and return a Boolean result. All relational operators are binary, because they compare two values. These operators are often used in conditional statements, covered in Chapter 4, JavaScript Fundamentals. Here is the complete list of JavaScript’s relational operators:
Table 7-9. Relational operators. Syntax == != < <= > >= Name Equals Not equal Less than Less than or equal to Greater than Greater than or equal to

String operands are compared according to the ASCII value of their characters. This comparison is similar to the one by which words are placed in a dictionary. The only difference is that instead of using a single set of letters, JavaScript uses 256 different characters. As you know, each character has a numeric value. This value determines if a character is greater than, equal to, or less than another one. When you compare multicharacter strings, the first character of the first operand is compared with the first character of the second operand. The second characters are compared only if the comparison of the first ones indicates equality. The process continues until the corresponding characters are not equal, or until the end of one of the strings is reached. Here are some characters and their ASCII values: 0 – 48 9 – 57 A – 65 Z – 90 a – 97 z – 122

Equality and Inequality
As you can see, the Boolean equality operator (==) is similar to the assignment operator (=). This similarity is the source of many programming errors, not only in JavaScript but also in other languages such as C, C++, and Java. Equality operators are often used in if-else statements, where the assignment operator cannot be used. (JavaScript does not allow side effects in a conditional statement.) In this case the interpreter produces a meaningful error saying: “test for equality (==) mistyped as assignment (=)? Assuming equality test.” However, in some other situations, both the equality and assignment operators are valid and the browser, rightly so, does not generate any errors. This is the reason why such errors are very difficult to debug. The following example demonstrates a situation in which both operators are acceptable:
var i = 1 document.write(i = 2) var j = 1 document.write(j == 2)

The first printing statement prints 2, because an assignment expression evaluates to the assigned value. The second printing statement prints false under Netscape Navigator, and 0 on Internet Explorer (Internet Explorer converts the Boolean value false to 0 for printing, rather than to "false"), because 1 is not equal to 2. In a long script, if you accidentally replaced one operator with the other, you would have a hard time finding the mistake. Another problem with the equality operator has to do with the way a computer deals with floating-point numbers. This problem is especially difficult when you try to store integer numbers in the form of floating-point numbers (e.g., 3.0). Depending on the

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underlying machine, the results may differ. You might find out that (5. == 5.00) evaluates to false, and (5.0 == 4.99999999) evaluates to true. Therefore, avoid using equality operators with floating-point values. The solution for this problem is presented later in the chapter.

Short-Circuit Logical Operators
Short-circuit logical operators, also called Boolean operators or logical operators, are binary operators that accept Boolean values as their operands. They group multiple relational expressions together. There are three logical operators:
Table 7-10. Logical operators. Syntax || && ! Name Logical OR Logical AND Logical NOT Type binary binary unary

Logical OR
operand1 || operand2

The logical OR operator accepts two Boolean values as its operands and evaluates to a single Boolean value. It evaluates to true if at least one of the operands is true; in other words, the only situation in which it evaluates to false is when both operands are false. Take a look at Table 7-11.
Table 7-11. Logical OR truth table. Operand1 true true false false Operand2 true false true false Operand1 || Operand2 true true true false

Be careful not to confuse the logical OR operator with the bitwise OR due to their similar meaning and syntax (|| vs. |). The only difference in meaning is that the first handles Boolean operands while the latter handles numeric ones. The logical OR operator is short-circuit evaluated. As you can see from the truth table, if the first operand is true, it is certain that the expression will evaluate to true. Checking the second operand in such a situation is unnecessary and is avoided by JavaScript.

Both operands must be true for the expression to evaluate to a true value. This operator is similar to the bitwise AND operator, except that the operand types are different, and the logical AND operator operates on the whole operand at once, rather than on a small segment (bit). Logical AND expressions are also short-circuit evaluated. If the first operand is false, the expression will obviously be false, so the second operand is not evaluated. Here are some examples:
2 > 1 && 3 < 4 // true 1 == 1 && 99 >= 98 // true "mouse" > "elephant" && 6 < 5 // false 1 == 2 && 5 <= 4 // false

Logical NOT
!operand1

The logical NOT operator is a simple unary operator which accepts a Boolean value and negates it. This operator is similar to the negation operator, which negates a number (changes its sign). It is even more similar to the bitwise NOT operator (~) which converts all 1 bits to 0 bits and 0 bits to 1 bits. Although the truth table is obvious, here it is for your reference:
Table 7-13. Logical NOT truth table. Operand true false !Operand false true

Because this is a unary operator, short-circuit evaluation is not relevant. Here are some examples:
!true // evaluates to false !(2 > 4) // evaluates to true

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!(7 == 7) // evaluates to false !false // evaluates to true

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Caution: Short-circuit evaluation is mostly useful because it makes your scripts more efficient. However, there are times that this method of evaluation is harmful. For example, you finish writing a complicated script and you want to test it. Depending on certain decisions in the script, short-circuit evaluation is performed, and the interpreter ignores the second operand of various expressions. You might think that the script works perfectly fine, even if there is a data mistype or a syntax error in the second operand. This type of evaluation can mislead you in such situations, but it is unavoidable.

Testing Equality for Floating-Point Numbers
We mentioned earlier that the equality operator (==) is not suitable for floating-point numbers due to inaccuracy issues. Using logical AND and OR operators, you can check if the number is close to the specified value. For example, if you want to evaluate the expression x == 10.0 you can use a fuzzy comparison using one of the following expressions, in which represents a small number (such as 0.001).
(x – 10.0) < || (10.0 – x) <

or
x > (10.0 – ) && x < (10.0 + )

More Logical Operators
The conditional operator and the comma operator are logical operators.

Conditional Operator
condition ? trueAlternative : falseAlternative

The conditional operator is unique because it is trinary (takes three operands) and because it returns values of all types. It can return a numeric value, a string, a Boolean value, and so on. The first operand is the condition. The condition must be an expression that evaluates to a Boolean value, either true or false. The second operator holds the value that the operator should return if the condition is true. The third operand is the value that the expression evaluates to if the condition is false. The conditional operator is often used with an assignment operator. For example:
var level = (points > 500) ? "Second Level" : "First Level"

The variable level is assigned either "First Level" or "Second Level", depending on the value of the Boolean expression points > 500. If the value of points is greater than 500, the conditional expression evaluates to the string "Second Level", which in turn is assigned to the variable level. If the value of points does not exceed 500, the

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string "First Level" is assigned to the variable. The first operand (the condition) must be Boolean (a single Boolean value or an expression that evaluates to a single Boolean value). The other operands can be of any type.

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Comma Operator
operand1, operand2, operand3, ...

The comma operator is rarely used. You can use it to force the evaluation of a set of expressions. The comma operator is also called a parameter delimiter because it does just that. You probably recall that we used the comma operator in functions when we wanted a function to accept multiple arguments. In this example the comma operator delimits the method’s arguments:
var beerNum = 99 document.write(beerNum, " bottles of beer on the wall")

Here is another example:
var a = (b = "Hello", alert("Hi"), "Howdy")

The comma operator forces the evaluation of all expressions in the statement. Only the last expression is returned, so the value of a would be "Howdy". This statement is equivalent to the following set of statements:
b = "Hello" alert("Hi") var a = "Howdy"

Data Type Operator
typeof operand1

or
typeof (operand1)

JavaScript provides an operator to check the data type of its operand. The operand can be either a literal or a data structure such as a variable, a function, or an object. The operator returns the data type. The expression includes the word typeof followed by the literal or identifier. Here are some examples:
typeof typeof typeof typeof typeof typeof // all foo == "undefined" // when foo is undefined eval == "function" // eval is a built-in function null == "object" // null is an object 3.14 == "number" true == "Boolean" "a string" == "string" of the expressions are true, of course

The typeof operator is very useful for debugging. Until strong debugging tools are available, you must do all debugging by hand, and detecting the data type of a structure is sometimes essential.

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Void Operator
void operand1 void (operand1)

or
javascript:void operand1 javascript:void (operand1)

The void operator, like typeof, is quite extraordinary. It specifies an expression to be evaluated without returning a value. Take a look at the following script:
function foo() { alert("Function entered") return true } alert(foo())

The preceding script segment displays two alert boxes with the following strings: n Function entered n true Now take a look at another function and call:
function foo() { alert("Function entered") return true } alert(void foo())

This script also generates two alerts, but the second one reads “undefined” because the void operator evaluates the function without returning a value. A more important use of this operator comes with hypertext links, where it is used to evaluate a JavaScript expression. The expression is evaluated but is not loaded in place of the current document. The following link does nothing because the expression 0 has no effect in JavaScript:
<A HREF="javascript:void(0)">Click here to do nothing</A>

The following code generates an alert box when the link is clicked:
<A HREF="javascript:void(alert('Wow'))">Click here to display message</A>

The parentheses are optional, so it’s up to you to decide whether to use them. Some scripters specify them in HTML and omit them in JavaScript for no good reason.

Expressions
The term expression has been mentioned dozens of times throughout this chapter. An expression is any valid set of literals, variables, operators, and other identifiers that evaluates to a single value. The value may be a number, a string, or a Boolean value. Conceptually, there are two types of expressions: n Those that assign a value to a variable (or another data structure) n Those that have a value The following are expressions:
a = "Dog" "Netscape" 256 false b = true

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The first type of expression is a bit more difficult to understand. In such expressions, an assignment is performed. The entire specification, including both operands of the assignment operator and the operator itself, is evaluated to the assigned value. Consider the following statement:
document.write(x = "Texas")

This statement prints Texas, because the whole expression evaluates to the assigned value. The next section discusses side effects, which take advantage of such expressions.

Side Effects
A side effect is an operation performed in addition to the main one. Take a look at the following statements:
number = 3 answer = ++number

The first line is a simple assignment statement. The second line is a bit more complicated. It performs two actions in the following order: n Increments number (side effect) n Assigns the value of number (4) to answer (main operation) Remembering the order in which the actions take place is not necessary. You could break down the side effect and use two different statements instead:
number = 3 number++ answer = number

This set of statements is easy to follow. The only “advantage” of the first method is that the code is compact. Compact code is a holdover from the early days of computing, when storage was expensive and programmers used various compacting techniques to save disk space. The situation is different today, because the number one rule in programming is to keep programs clear and simple. It comes even before efficiency, especially in simple programs like the ones written in JavaScript (you will not find any 500,000-line scripts in this book). Other side effects are not related to the increment and decrement operators. Consider the following JavaScript statement:
document.write(myName = "Chuck")

You already know that this statement prints Chuck. The assignment operation also takes place in this statement, so myName holds the value Chuck after the statement. The assignment operation is the side effect, and the printing action is the main action. The preceding statement should be split into two separate statements:
myName = "Chuck" document.write(myName)

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Here are some more statements you should avoid (they will keep you busy for a while...):
number = 0 answer = (number++ – 2) * (--number – 1) document.write(number = answer++)

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This set of statements prints 2. Let’s analyze it:
1. 1.1 2. 2.1. 2.2. 2.3. 2.4. 2.5. 3. 3.1. 3.2. 3.3. number is assigned the value of answer (2). The value of number is printed. answer increments to 3. (number++ – 2) is evaluated to –2. number is incremented to 1. number decrements to 0. (--number – 1) evaluates to –1. answer is assigned 2 (because (–1) * (–2) yields 2). number is assigned the value 0.

The final result: n number holds the value 2. n answer holds the value 3. n The number 2 is printed to the document. The following script is longer, but it is definitely better:
number = 0 answer = number – 2 number++ number-answer *= number – 1 number = answer document.write(number) number = answer answer++

Summary
In this chapter we discussed JavaScript’s wealth of operators. Some of them are extremely useful, while others are not used at all. We also discussed various expression forms. In addition, we played around with some side effects. Never use side effects in your programs, because they are hard to follow and difficult to understand. The next chapter analyzes a JavaScript script.

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JavaScript by Example— A Review
In this chapter we will examine a complete JavaScript script. The purpose is to ensure that you can write a real-world JavaScript script. It is vital that you fully understand the material in the first eight chapters before continuing with the rest of the book. So be sure to carefully review this chapter.

Height and Weight Calculator
The Task
This script takes the user’s height and weight, and displays a greeting that includes a comment on the user’s weight status. The comment is based on the height and weight measurements of the user, the proportion between them, his or her age and sex, and an overall result. It optionally displays “scientific” results, which include height and weight percentile and their quotient. The purpose of this task is to provide a quick, working review of what you have learned about JavaScript.

The Algorithm
The script first requests the user’s age and sex. If the age exceeds 18, it is converted to 18 in order to fit the pre-entered database. Then, the script asks the user to select a measurement system: metric or English. The selected units are reflected in the messages asking the user for his or her height and weight (e.g., “Enter height in centimeters:”). If the user chooses English, the weight and height are converted to the metric system (cm, kg), after which their percentiles are calculated. Based on the user’s age, the script finds the height and weight of the 5th and 50th percentiles. Using a simple interpolation, the user’s specific height and weight percentiles are calculated. The script checks that the computed percentile is between 1 and 99, and out-of-range results are converted to the nearest valid value (1 or 99). JavaScript displays a comment according to computed percentiles. The script also gives the user an option to view the more “scientific” results, including the actual numeric percentiles

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and their quotient. The weight and height percentiles were taken from Compton’s Encyclopedia.

Output
Using some sample input data, the output looks like the following series of images:
Figure 8-1. A dialog box that prompts the user to enter his or her sex.

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Figure 8-2. A dialog box prompting the user to enter his or her age. Figure 8-3. A dialog box that prompts the user to specify measurement. Figure 8-4. A dialog box prompting the user to enter his or her height. Figure 8-5. A dialog box prompting the user to enter his or her weight.

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Figure 8-6. A short sentence based on the collection of calculations. Figure 8-7. A dialog box that allows the user to view the “scientific” results.

Figure 8-8. The “scientific” results.

Figure 8-9. A disclaimer.

Analysis
The script looks rather long, but it is simple and easy to understand, because much of it is pure data and a repeated if-else statement. convertInput() The convertInput() function does not accept any arguments; it simply modifies two global variables. The function converts weightInput and heightInput from the English system to the metric one. weightInput is converted to kilograms, and heightInput is converted to centimeters. The function does not return any value because both variables are global. getHeight(sex, age, height) This function accepts three arguments: the user’s sex, age, and height. The variable height, holding the user’s height, is rounded off to the nearest integer. The height is already represented in centimeters, even if the user chooses to use the English system.

Let’s examine this script segment to see what it does. At first, JavaScript checks if age is equal to 2. If it is, the value 87 is assigned to height50 and 82 is assigned to height5. These are the approximate values from the graph. If age is not equal to 2, the expression:
age == 2

evaluates to false, and JavaScript carries out the else statement. Another part of the function is the section where the female statistics are different from the male ones:
if (age == 11) { height50 = 143 height5 = 131 } if (sex == "f") { if (age == 12) { height50 = 150 height5 = 140 }

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After rounding off the height (age is already an integer, rounded off by a global statement), two new local variables are declared and initialized with the numeric value 0: height50 and height5. The variable height50 is intended to hold the average height measurement for a male or female of the user’s specified age. The other variable, height5 holds the height of the 5th percentile. We use an if-else construction to assign these two variables the appropriate values. Each if statement checks if the user’s age is equal to the age in the condition expression (e.g., age == 10). The else statements instruct JavaScript to continue executing the if statements in order, as long as the value of age does not match the value against which it is tested. From a quick look at the graph, one can find that males and females have the same height until age 11. Therefore, the first 11 if statements apply to both female and male users. Here is a short section from the function:

An if statement checks if the user is a female. A true condition instructs JavaScript to execute a command block containing the female-specific if-else segment. JavaScript ignores the command block if the condition is false, i.e., the user is a male. The next area of the function is specific for males. The entire if-else execution terminates after the user’s age is matched. The maximum age inside the function is assumed to be 18. If the user enters a higher number, it is converted to 18 in the global section of the script before this function is invoked. Finding the y (percentile) value of a given x (height) value is a problem you have probably solved in math classes: percent = (height – height5) * (50 – 5) / (height50 – height5) + 5. The script uses this exact formula. getWeight(sex, age, weight) This function is the same as the previous one, except that it deals with the user’s weight instead of his or her height. printResult(height, weight, sex, age) This function is more interesting than the previous ones because it does not have any repeating segment. At the beginning of the function, five adjectives are declared and initialized to an empty string—a clue that they are used to hold string values later in the function. Another variable, grade, is declared and initialized to 0, indicating that it is used to hold a numeric value. It is especially important to assign the value 0 to this variable because it is accumulative, and any other value would generate wrong results. A new variable named propWeight is declared and initialized to the user’s weight percentile, divided by his or her height percentile. This measure normalizes the person’s weight by his or her height, giving a more accurate picture of the person’s physical appearance (you can’t expect an 8-foot person to be of average weight, can you?!). The

The preceding script segment assigns one of three values to the variable heightAdj: "tall", "short", or "medium-height". If the height percentile is over 70%, the person is considered tall; if it is below 30%, the person is considered short. In all other cases (30% to 70%), the selected adjective is "medium-height". Notice that along with the adjective, a number is added to the variable grade, representing the number of points awarded to the given characteristic. For example, a tall person receives two out of three possible points in the height category. Medium-height people receive the maximum three points, whereas short people receive the minimum one point. All of this is done with a nested if-else structure, where each statement is a command block consisting of two nested statements. WeightAdj is assigned similarly. The adjective describing the person’s age is created by concatenating his or her age to the string "-year-old ". Notice the number to string conversion (cast) in this statement. An overall adjective is assigned to gradeAdj according to the final grade. (Do not take any result as an insult or as medical advice, because it almost never returns a realistic one.) Finally, the user’s sex is assigned to sexAdj, according to his or her input at the beginning. All adjectives are now ready, so a comment is constructed with the following format:
"You are a " + heightAdj + "," + weightAdj + ", " + gradeAdj + ", " + ageAdj + " " + sexAdj + "."

An alert box then displays the sentence. JavaScript displays another alert box with the “scientific” percentile, weight, and height results, if the user wishes to see that information. This is done via an if statement and a confirm() method as its condition. The last operation in this function is the disclaimer-like message.

At first, JavaScript asks the user to enter his or her sex as a single letter (e.g., “f”, “m”). Notice that none of the input values in the script are evaluated, so the user must enter a valid value. Data evaluation is discussed later in the book. JavaScript then asks the user for his or her age, and converts it to a number (all input from dialog boxes’ fields are in the form of strings) with the parseInt() function. This built-in function is also discussed later in the book. The user’s age is rounded off to the nearest integer. The user is then prompted to choose the desired measurement system: metric or English. The appropriate values are assigned to heightUnit and weightUnit (e.g., centimeters, inches, kilograms, pounds) via a conditional expression. Before calling any function, JavaScript asks the user for his or her height and weight. Each value is converted to a number using the parseInt() function. At this point, the height and weight values are converted to metric units, via convert(), if they are in English units. The value of the variable age is assigned 18 if it is greater than 18, and 2 if it is less than 2. These values are the limits, and out-of-range values would not work with the getHeight() and getWeight() functions.

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The getHeight() and getWeight() functions are called in order and return the computed percentile. The percentile is cut to its limits if it is out of range. The minimum percentile is 1; the maximum is 99. The last statement calls printResult() to print the results as explained earlier.

Summary
In this chapter, we have explained in detail a complete JavaScript script. This script is a good summary of the first part of the book, and you should take the time to comprehend it in depth. It demonstrates the basic features of the language such as variables, variable scope, conditional and loop structures, functions, alert boxes, forms, prompts, output techniques, data type conversion, and more. The scripts in the remainder of the book are much more complicated, so make sure your knowledge base is as broad as these scripts entail.

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Events in JavaScript
Events
Not long ago, static HTML pages ruled the web. However, as more web sites use dynamic and interactive content, users are becoming accustomed to this more exciting format and are less interested in static web pages. HTML provides a limited set of features. When browsing an HTML-based web page, you can read the text, look at the graphics, or possibly listen to the sound it plays. All of these actions are static in that you as a user have no control over the actions the web page takes. Such pages are similar to art masterpieces in a museum. You can look at them, maybe take a picture, but you cannot interact with the paintings. For many people the web experience consists of only visiting pages, without interacting with them. The only interaction experienced is when the user follows a link or clicks somewhere on an image map. JavaScript enables the designer of an HTML page to take advantage of events for automated navigation, client-side processing, and more. Events are signals generated by the browser when various actions occur. You will see later that JavaScript is aware of these events and can react to them. The events supported by JavaScript are listed in Table 9-1.
Table 9-1. Events in JavaScript. Event abort blur Description Occurs when the user aborts loading an image. Occurs when focus is removed from a form element (when the user clicks outside the form element). It also occurs when the user removes focus from a browser window. Occurs when the user clicks on a link, an image map area, or a form element. Occurs when the user changes the value of a form field. Occurs if there is an error loading an image. Occurs when the user gives input focus to a form element or a window. Occurs when a page or image has finished loading into the browser window.

click change error focus load

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Event mouseOut mouseOver select submit unload

Description Occurs when the user moves the mouse pointer from inside a link or image map area’s bounding box to its outside. Occurs when the user moves the pointer over a hypertext link or an image map area. Occurs when the user selects a form field. Occurs when the user submits a form via the Submit button. Occurs when the user exits a page.

Not all actions that take place in your browser are events. Events are only actions that occur in the area of the HTML page, such as loading images and selecting form elements. If an action occurs directly in the boundary of the browser’s display zone it is an event as well. Take a look at the following page:

You know that this window consists of two parts: the HTML page and the browser. You also know that many actions can occur in the environment illustrated in Figure 9-1. Some are events while others are nonevent actions. The main part of the window is the HTML page, the content window without the surrounding menus and buttons:

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Figure 9-2. Netscape’s home page without the surrounding browser’s window.

All actions that take place in the area shown in Figure 9-2 are events. The user loads, unloads, clicks links, and causes many other events in this zone. The second section consists of Netscape Navigator’s features, such as the menu bar, the scroll bar, and other elements seen in the following image:

Figure 9-3. Netscape Navigator’s window with no content zone.

This window is exactly the same as the one presented in Figure 9-1, except that the HTML page is excluded. No events occur outside of the HTML page. For example, opening Navigator’s mail or news window does not generate an event. Scrolling up and

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down the window does not cause an event either. Even if you change the appearance of an HTML page, you are not triggering an event. It is very important to realize the difference between events and other actions because JavaScript can only handle events, not other actions. Many questions in mailing lists and news groups such as “When do the people at Netscape plan to create an event handler triggered by scrolling...?” show the ignorance of programmers in recognizing what an event is. An event handler can obviously handle only events! Events occur no matter what the size of the HTML page zone is, as long as a page is loaded. Note that future releases of Navigator or Internet Explorer may expand the term of events to the entire window, but that is still in doubt. Netscape Navigator 3.0x made a step towards this when it introduced the ability to automatically scroll via JavaScript, though a corresponding event was not implemented.

Event Handlers
Event handlers correspond to their associated events. They are scripts that execute automatically when events occur. Most deferred JavaScript functions are called by event handlers. You saw earlier that events are actions that do not have any direct influence. They only serve the event handlers. Each event handler is associated with an event. Event handlers are embedded in documents as attributes of HTML tags to which you assign JavaScript code. The general syntax is:
<TAG eventHandler="JavaScript Code">

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You can use event handlers to embed any JavaScript code. You can place a 500-line script including functions as an event handler tag. You can use more than one event handler with the same HTML tag. The names of the event handlers are constructed of the word “on” and the name of the event. Here is the full list of supported event handlers: n onAbort n onBlur n onClick n onChange n onError n onFocus n onLoad n onMouseOut n onMouseOver n onSelect n onSubmit n onUnload

Example 9-1 (ex9-1.htm). An HTML document with two event handlers as separate attributes of the same tag.

Event handlers are not case sensitive. For example, you can use ONLOAD instead of onLoad. Although event handlers are related to JavaScript, the event handler itself is normally used as an HTML attribute—HTML is not case sensitive. It is still a good practice to use identifier-like naming conventions for event handlers. As you will see later, the official name of an event handler is all lowercase, but in HTML it doesn’t matter. Example 9-2 is a classic script that takes advantage of event handlers.
<HTML> <HEAD> <TITLE>A link description in the status bar</TITLE> </HEAD> <BODY> <A HREF="http://www.wordware.com/" onMouseOver="window.status = 'Wordware Publishing'; return true">Wordware</A> </BODY> </HTML>

Example 9-2 (ex9-2.htm). An event handler script that displays a short description of a link when the user has the pointer over that link.

It is sometimes problematic to execute statements directly in an event handler script. For example, if you want to include a long script, placing it in the event handler script makes the HTML page cumbersome, and maintenance becomes difficult. Another case in which difficulties can arise is when you want to correlate a script containing strings—quotation marks—to an event handler. As you can see, the event handler requires quotation marks to delimit the specified JavaScript code from the surrounding HTML content. JavaScript requires alternation of quotation types, single and double, so you will probably find it annoying to write scripts with an emphasis on quotes. Event handlers accept any JavaScript script, as long as it is valid. For that reason, you should associate functions with event handlers. The only statement you need in the event handler script is the function call. You should normally place all functions at the top of the page, or more accurately, in the <HEAD>…</HEAD> portion of the document. This action forces JavaScript to evaluate your functions before it continues laying out the page. With this practice you guarantee that when the browser comes across an event handler in the HTML portion of the

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page, it will succeed in calling the function associated with that event if it occurs. The only restriction is that you cannot place any event handlers to pick up events before the JavaScript script in the <HEAD>…</HEAD> portion. Such a restriction applies also to external files implemented via the SRC attribute of the <SCRIPT> tag. The basic structure of such a page is:
<HTML> <HEAD> <TITLE>The title of the page</TITLE> <SCRIPT LANGUAGE="JavaScript" [SRC="path.js"]> <!-function functionName(parameters) { statement1 statement2 statement3 } // --> </SCRIPT> </HEAD> <BODY> <TAG otherAttributes eventHandler="functionName(arguments)"> </BODY> </HTML>

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It is very easy to find the associated functions if you always place their definitions in the <HEAD>…</HEAD> portion of the page. The HTML portion of the page stays as simple as it was before JavaScript was invented. The only new piece of HTML is the event handler. Another important advantage of using functions with event handlers is that you can use the same function many times in the same HTML page. For example, if you have a form with four text fields, you can validate each one of them using the same function by calling this function upon occurrence of an event which is specific to one of the fields. You may have noticed that the title is specified before the <SCRIPT> tag. Although the title is specified via an HTML tag, it has no event handlers that could possibly invoke a function defined later. You will probably not have any problems if you do not follow this rule, although it is still a good practice, because it contributes to the neat organization of the page and guarantees that you will never have any layout problems.

onLoad and onUnload
Two very important event handlers are onLoad and onUnload. Their corresponding events, load and unload, are triggered by those actions. When the page has finished loading, the load event takes place. When the user exits a page in any way, the unload event occurs. These are the most simple event handlers because they are related to the most basic <BODY> tag and are specified as attributes of this tag. The following document welcomes the user via an alert box when entering the page, and says goodbye when the user exits, also via an alert box:
<HTML> <HEAD>

The onLoad event handler is widely used to call deferred scripts. Placing a function call as the event handler’s script enables you to control the timing of the execution, so the script executes only when the page is fully laid out.

onError
The onError event handler expands JavaScript’s ability to interact with the user depending on errors that occur. This event handler is not supported by Navigator 2.0x or IE 3.0x, but is supported by the latest version of both browsers. An error event occurs when the loading of a window or image causes an error. The onError event handler executes a JavaScript script when such an error occurs. The onError event handler is extremely useful due to the fact that it can be set to one of three values: n null—suppresses all error dialogs. Setting window.onError to null means your users won’t see JavaScript errors caused by your own code. n A function that handles errors—replaces the standard dialog boxes used by JavaScript to report errors. n A variable that contains null or a valid function reference. The following JavaScript statement disables JavaScript error dialogs:
window.onError = null

You should place this statement in its own script directly after the <HEAD> tag to be on the safe side, using the following HTML document structure:
<HTML> <HEAD> <SCRIPT LANGUAGE="JavaScript"> <!-window.onError = null // --> </SCRIPT> </HEAD> <BODY> </BODY> </HTML>

The same rule applies to all values you assign to this event handler explicitly. The following document does not display any error dialog, although an error is encountered. (If you can’t figure it out, you may want to review the previous chapters, or just look at the alert() method’s argument for a clue.)

It is a good practice to set the onError event handler to null when your script runs on a public page. However, it makes no sense when writing the script, because it makes debugging impossible. Here is another document that disables errors and therefore avoids a stack overflow error from being reported:
<HTML> <HEAD> <TITLE>Disabling error messages</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-// disable error reports window.onError = null // create an error-generating function (infinite recursion) function testErrorFunction() { testErrorFunction() } // --> </SCRIPT> </HEAD> <BODY onLoad="testErrorFunction()"> </BODY> </HTML>

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An error occurs with or without assigning null to the event handler. The difference is that no response to the error on the browser’s behalf is generated. See “Calling Event Handlers Explicitly” later in this chapter for a discussion on assigning values to event handlers via JavaScript. Another option is to write a function to handle errors in place of the standard JavaScript error-reporting dialog boxes. The function should accept three arguments: n The error message n The URL of the script that caused the error n The error line number

Example 9-3 (ex9-3.htm). A set of functions to intercept and display a list of errors.

At first, the function reference is assigned to window.onError. Three arrays are then created; each holds a different piece of data associated with errors that might occur. The length of each array is set to 0, because no errors have occurred thus far. The function that intercepts the JavaScript errors is myOnError(). Its arguments are the error message, URL, and line number, respectively. You do not need to understand the functions at this stage because they use concepts that have not been discussed yet, such as arrays and windows. The only point you should be aware of is that the function returns true to instruct JavaScript that it is intended to intercept the standard JavaScript errors. In general, a list of the JavaScript errors generated by the script is printed in another window when the user clicks the button. See the section “Calling Event Handlers Explicitly” for more details on the event handler implementation demonstrated in Example 9-3 (window.onError).

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Emulating Events via Methods
We mentioned earlier that each event belongs to a specific object. Some events are associated with more than one object, especially since the release of Netscape Navigator 3.0, which expanded certain events to additional objects. Another characteristic of objects is methods. They are functions that usually work on the data related to that object, the properties. Some methods of objects that include event handlers actually emulate those events. You can use such a method to cause an event to occur. These methods are usually called event methods. Although we will discuss these methods in depth later, here are some of them: n blur() n click() n focus() n select() n submit() Events generated with these methods are like any other method. Most importantly, they do not invoke their corresponding event handlers. When you emulate an event, it is important that you do so only after the browser has finished laying out the page, or at least the object (usually a form element) with which the method is associated. The following page generates an error:

Error Message Content
This error is not guaranteed to be exactly the same on all platforms. Emulating an object’s event that has not yet been laid out is just one example that creates such an error. Generally speaking, you cannot refer to any element of a page that has not yet been laid out. A deferred script is allowed to refer to an object laid out after the script, provided that you do not execute that script before the object has been laid out. We will discuss this issue in depth throughout the book.

Example 9-4 (ex9-4.htm). You can call a JavaScript script via an event handler.

Try loading this page and clicking the button. If you are using Navigator 3.0x or 4.0, an alert box displays the message “This is the second function,” despite the fact that the event handler associates the click event with the first function, alert1(). Notice the script placed directly after the form. It associates a different function with the click event of button1 in form1.

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Note: Event handlers are function references as opposed to function calls. Therefore, you must assign alert2, not alert2(), which is primarily a function call, evaluating to the type and value the function returns. Also, since the event handler HTML attributes are literal function bodies, you cannot use <INPUT … onClick=alert2> in the HTML source. You must call the function instead.

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You are probably wondering why we used onclick rather than onClick. The reason is that JavaScript is case sensitive and understands onclick (all lowercase), whereas HTML is case insensitive. In HTML, you may use all lowercase, all uppercase, or any other convention you choose. Calling event handlers explicitly enables flexible event handlers, because you can modify them anywhere in the script. You can create a customized page, allowing the user to determine what a certain button should do when clicked. An event handler does not have to exist when you use this technique. The preceding script would work exactly the same if you omitted the event handler specification in the HTML source and used the following line instead:
<INPUT TYPE="button" NAME="button1" VALUE="button1">

In this case you are actually defining a new event handler to correspond with an event. Also note that the above method is the only way to assign a function reference to an event handler. You cannot use any of the following structures:
<SCRIPT LANGUAGE="JavaScript"> <!-document.form1.button1.onclick = "alert2()" // --> <SCRIPT LANGUAGE="JavaScript"> <!-document.form1.button1.onclick = alert2() // -->

Such attempts to assign statements that are not function references result in an error upon clicking the button.

Cancelling Events
Netscape Navigator 3.0x and 4.0 make it possible to cancel a click event, either with a hypertext link or a client-side image map. The onClick event handler can return false to cancel the action normally associated with a click event. Here is an example:
<HTML> <HEAD> <TITLE>Cancelling events in onClick</TITLE> <BODY> <A HREF="http://www.netscape.com/" onClick="return confirm('Load Netscape home page?')">Netscape</a> </BODY> </HTML>

Example 9-5 (ex9-5.htm). Try running this script to cancel the event.

When you click the hypertext link, a confirm box pops up. If you click OK, the method evaluates to true, the event qualifies, and the new page (Netscape’s home page) loads. If the user selects Cancel, the method returns false, and the new page, associated with the click event, does not load.

Summary
In this chapter you discovered events and event handlers. JavaScript adds interaction to web pages, meaning you can respond to actions that the user performs. Events describe such actions, whereas event handlers take care of them. They give JavaScript programmers the ability to interact with the user. Only rarely do you find scripts on the web that do not take advantage of this unique feature. Although interaction includes more than event handlers (we’ve seen dialog boxes before), they are probably the most important means of interaction in JavaScript. They extend the capability of web pages, as you might have noticed while surfing the net. We did not present any events or event handlers in detail in this chapter, except for onLoad and onUnload, because they will be discussed individually in depth later in the book. Each event and its corresponding handler has significant features. As you will see later, all events are associated with specific objects, so it is important that you be introduced to these objects before we discuss their event handlers.

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Building and Extending Objects
Creating Instances
Objects are templates by which you create instances. For example, suppose you have defined an employee object that includes some methods and properties of an employee, such as his or her social security number and address. This definition has no effect on the script because no new entity has been changed or created. Only when you apply the object’s definition to a specific person, say John Doe, is a new entity created—an instance of that object. The number of instances you create with a single object definition is unlimited. The object definition can be either one you define with a function or a built-in one. Creating an instance of a built-in object is relatively simple because the object is predefined. This is a bit different than Java, C++, and other object-oriented languages where a “class” is the template code and a specific instance is an object. You should use the following syntax to create an instance:
var name = new objectname([arguments])

So, if you want to create an instance of the String object, you can use the following statement:
var str1 = new String("Hello!")

This statement creates an instance named str1 and passes it the literal “Hello!” You can create another instance if you like:
var str2 = new String("Hi!")

Most objects, String included, have what is called a constructor function. That is a function that fires automatically when an instance of that object is created. When you pass a parameter to an object when creating an instance you are actually passing that parameter to the object’s constructor function. In the previous example the instances str1 and str2 act pretty much like regular variables. In fact you can think of them as variables of the String type. You can pass them on to functions as in:

Remember that if you create an instance of an object inside a function, it is considered local, i.e., it is visible only inside the function during the function’s current execution course. If you declare the instance outside a function, it is preferable that you use the var keyword. When you create an instance of an object inside a function without var, it is a global structure. Bear in mind that if you create a global instance, or even a simple variable inside a function, you must execute the function before that global data structure exists in the script. From that point on you can refer to it freely anywhere in the document. When you create an instance of an object via the new operator, you are actually declaring a specific data type according to the object’s definition and allocating the appropriate amount of memory for that data type. All properties referenced in the constructor function are accessible as properties of the object’s instance. After you have created an instance of an object, you do not have to use the keyword new anymore when referring to that instance. However, if you want one of the instance’s properties to be an instance of its own, you must use new again to create the new object. An instance of an object also features its methods. A method is essentially a function that is found inside of an object. Take a look at the following example:
var current = new Date() // the current date var minutes = current.getMinutes() var current2 = new Date() // the current date var minutes2 = current2.getMinutes()

As you can see, both instances have methods. A method belongs to a specific instance of an object. In other words, the getMinutes() associated with current object is a separate function from the getMinutes() associated with the current2 object. Both functions are made from the same template and perform the same action.

Constructor Functions
A constructor function is a simple function that defines the properties and methods of the relevant object type. It fires automatically when an instance of the object is

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created. You can think of built-in objects as objects whose constructor functions are predefined in JavaScript, so you do not need to write them on your own. A constructor function resembles a cookie cutter. You provide it some dough and it gives the dough the proper shape. The cookie cutter is like the constructor function because they both receive a simple structure and change it according to a specified template.

Defining Object Properties
The keyword this is probably the most important word related to objects in JavaScript (as well as many other object-oriented languages). It refers to the current object, or instance. Inside a constructor function it refers to the instance for which the function was called. Take a look at the following function:
function student(name, age, avgGrade) { this.name = name this.age = age this.grade = avgGrade }

This function accepts three arguments. It defines an object type of a student in a class. The properties are name, age, and grade, and they are initialized by the values passed on to the function. You can use the following statement to create an instance of this object—a student in a class:
var student1 = new student("Sharon", 16, 85)

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Now you can refer to these properties in the following fashion:
alert(student1.name + " is a cute " + student1.age + " - year old.)

It is also possible to add properties to an object once it has been created. Such properties exist only in the specific instance to which they are assigned. The following script segment demonstrates this:
function student(name, age, avgGrade) { this.name = name this.age = age this.grade = avgGrade } var student1 = new student("Sharon", 16, 85) student1.sex = "female" var message = student1.name + " is a cute " + student1.age message += " - year old " message += (student1.sex == "female") ? "girl." : "boy." alert(message)

The problem with the preceding script is that it adds the new property only to one instance, student1. Because constructor functions are just like any other function,

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you can use valid JavaScript statements in them. Therefore, you can use the following constructor function to add the new property to all instances of the student object:
function student(name, age, avgGrade, sex) { this.name = name this.age = age this.grade = avgGrade this.description = (sex == "female") ? "girl" : "boy" }

Now you can create instances that will include the “girl” or “boy” description:
var student2 = new student("Joe", 16, 91, "male")

Based on the exact definition of the word this, some JavaScript tends to use alternative structures for construction functions. Here is the preceding example in a different form:
function student(name, age, avgGrade, sex) { obj = this obj.name = name obj.age = age obj.grade = avgGrade obj.description = (sex == "female") ? "girl" : "boy" }

Notice that the calling object, referred to as this, is assigned to a variable. This variable must be global because a constructor function’s purpose is to create an instance for use outside of that function. A local variable does not have any effect outside the function where it is declared. Personally, I never use this method and don’t see it used very often. I simply include it here so that you will not be confused if you see it somewhere. You now know how to define properties via constructor functions. If you only want to create one instance of an object, you must also use a constructor function. To view the properties of an object you can use the following function:
function getProps(obj, objName) { var result = "" // initialize output string for (var i in obj) { result += objName + "." + i + " = " + obj[i] + "<BR>" } result += "<HR>" // add horizontal rule to output string return(result) // return output string }

To invoke this method, use a statement such as:
document.write(getProps(student1, "student1"))

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Note that the arguments are not always the same. For example, if you use this statement from within another function that accepts the instance student1 as the parameter person, you should use the following statement instead:
document.write(getProps(person, "student1"))

If a property of an object holds a null value, it does not exist. Assigning a null value to a property will cause the function getProps() to count out that property. Keep this in mind, especially when you are debugging a script.

Nested Objects
JavaScript supports nested objects. A nested object is an object that is also a property of another object. There are a few ways to create nested objects. Take a look at the following script portion:
function student(name, age, grade, father, mother) { this.name = name this.age = age this.grade = grade this.parents = new parents(father, mother) } function parents(father, mother) { this.father = father this.mother = mother } var student1 = new student("Sharon", 16, 85, "Mark", "Stacy")

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This script consists of two functions. The function parents() creates an instance containing two properties, father and mother. It accepts both values as arguments. The first function, student(), creates a function consisting of four properties. The first three are simple properties like the ones you have seen earlier in this chapter. The fourth property is an instance of an object. This instance is created by the parents() function, as described earlier. You can refer to all elements of an object with the dot syntax:
student1.name student1.age student1.grade student1.parents.father student1.parents.mother

To print the properties of a general object you can use the following recursive function:
function getProps(obj, objName) { // initialize accumulative variable var result = ""

The function’s algorithm is fairly simple. It loops through the properties of the main object. If the current property, represented by i, is an object, the function is called once again with the property obj[i] as the object, and the property’s name attached to object’s name with a separating dot (objName + "." + i). Each call to the function returns the string listing the properties at the current level. The value returned by a recursive call is assigned to the variable result which is local in the calling function. Here is an entire HTML document and its output to help you understand this concept:
<HTML> <HEAD> <TITLE>Printing properties of nested objects</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function student(name, age, grade, father, mother) { this.name = name this.age = age this.grade = grade this.parents = new parents(father, mother) } function parents(father, mother) { this.father = father this.mother = mother } var student1 = new student("Sharon", 16, 85, "Mark", "Stacy") function getProps(obj, objName) { var result = "" for (var i in obj) { if (typeof obj[i] == "object") result += getProps(obj[i], objName + "." + i) else result += objName + "." + i + " = " + obj[i] + "<BR>" } return result }

Notice the extensive use of the keyword this inside the function to refer to the object. The main characteristic of a method is that it usually processes the data of its object. You can even create a “constructor method” in the following fashion:
function construct(name, val) { this[name] = val } function student(name, age, grade, father, mother) { this.construct = construct this.name = name this.age = age this.grade = grade this.parents = new parents(father, mother) } function parents(father, mother) { this.father = father this.mother = mother }

Notice that the method (construct()) refers to the new property via the array notation. You may recall from Chapter 5, Object-Oriented Programming, that you must use this notation if you wish to use a data value. You can only use the “dot” syntax when you use literals. In this case, name is a data structure, not a literal (a literal is a constant value you can see, such as "boyfriend"). Now you can use the recursive function presented earlier to view the elements of an object, and to see how it works with methods. Notice that the recursive function works with methods as well as with properties. The statements of the method are placed on the same line. JavaScript automatically adds semicolons to delimit the statement. It also uses a uniform coding scheme if you did not do so originally in the function, as you should. A method in JavaScript, like a property, belongs only to one instance. You can only invoke it from that instance. All “communication” with an object is done via methods. The calling object is considered global inside a method. You can modify the object using the word this.

Creating Objects—An Example
Let’s say you want to create a database-like object structure for a store that sells televisions. Each type of television should be an instance of the same object. The number of televisions of a certain model available in stock should be a property of the object. It should also include the features of that television set. In addition, the object should include two methods: one to be invoked when a customer buys a television set and another to be invoked when the store owner orders a certain quantity of television sets of a given model. Example 10-2 demonstrates the most important points.
<HTML> <HEAD> <SCRIPT LANGUAGE="JavaScript"> <!-function television(brand, size, price, num) { this.brand = brand // property this.size = size // property this.price = price // property this.num = num // property this.sell = sell // method this.buy = buy // method this.display = display // method } function sell(quantity) { if (quantity > this.num) alert("Not enough " + this.brand + " " + this.size + "\" sets in stock.") else this.num -= quantity

Example 10-2. A script that stores data using instances of a general object.

When you run Example 10-2 you should get the output shown in the following series of images:

Figure 10-2. A table showing the inventory of television sets.

Figure 10-3. An alert box that notifies the user if there are not enough sets in stock.

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Figure 10-4. An alert box that tells the user to order more sets.

Example 10-2 features three methods: n sell() This method modifies the number of television sets of the model number specified in the object. The function’s only parameter is one that accepts the number of television sets sold. It checks if there are enough sets in stock, because you cannot sell more than you’ve got! If there are enough sets of that type in stock, the number is subtracted from the total quantity of that TV available before the transaction. Lastly, the method checks if there are less than five TV sets of that type left. If so, it sends a message describing that situation.
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buy() This method simply adds the number of television sets to the num property of the object, or instance. The only parameter accepted by the method is the one that accepts the number of television sets bought. display() This method does not accept any arguments. It simply prints all the properties of the current object in a tabular format.

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Notice that the first two methods actually change values of the object. This is another example that shows that methods refer to their calling object as global, so all properties of the object modified inside the function (method) are also affected outside the method.

Prototype and Object Extensions
A prototype is a property of any object created with the keyword new. A prototype resembles the common e-mail address portion of users sharing the same server. Likewise, a prototype belongs to all instances of the object that the prototype belongs to. Prototypes refer not only to properties, but also to an object’s methods. Take a look at the following script segment:
var str1 = new String("a"), str2 = new String("b") function repeat(n, delimiter) { var text = "" var str = this.toString() // make sure the object is string while (n >= 0) { text += str text += delimiter n--

You can see that the function repeat() is typical for a method. As a matter of fact, it is implemented as one. The key statement in this script is:
String.prototype.repeat = repeat

This statement adds a method to all instances of the object (String), although they were already created. The word prototype is constant, and its role is to merely specify that the following property or method is a prototype. It may seem at first that repeat() is a method of a nested object. However, prototype is not actually a property but a specification. Here are some of the alert boxes generated by the preceding script segment:

It is important to know that object prototypes refer to instances of the object created before the definition of the prototype as well as instances that are created later. The prototype exists only from the moment it is defined. Be sure to refer to such prototype properties and methods only after you have created the prototype. Commenting scripts helps assure that you follow this rule. Up until now you have seen a method used as a prototype. Obviously, properties can also accomplish this task. In the following script we will look at a property as a prototype of a user-defined object. Here is the script:
function car(make, model, year) { this.make = make this.model = model this.year = year } var mine1 = new car("Hyundai", "Lantra", 1996) car.prototype.wheels = 4 // no 3-wheelers please! var mine2 = new car("Mazda", "Prodigy", 1996)

The message at the end of the script is printed, because all instances of the car object, whether they are created before the definition of the prototype or after it, consist of a property named wheels, which holds the value 4, that is assigned to the prototype. It is practically useless to create a data property prototype without assigning it a value, or at least a default one, because you will need to refer to that property later, and such reference automatically creates the property. If you run the script above in an HTML page, you should see something like Figure 10-5 in your browser.

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Figure 10-5. Output for the function car().

Prototypes are probably more useful with built-in objects, because they can modify the object. You can create outstanding methods and properties you never dreamed JavaScript would support. We use prototypes in many examples in this book, especially ones dealing with strings, dates, and arrays. Prototypes were not supported in Netscape Navigator 2.x, but their existence in Navigator 3.0x (and above) turned the JavaScript language upside down. The revolutionary concept is that you can add features to the language itself, almost as if you were one of JavaScript’s developers at Netscape Corp. or Microsoft (although MSIE 3.0x does not support this feature). Note that prototypes can only be added to objects that support instances, as opposed to static objects (such as Math) which do not work with prototypes for obvious reasons.

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Summary
In this chapter you learned how to create instances of an object. Instances refer to user-defined objects as well as to built-in ones. You also learned about constructor functions that build objects or object types. As you will see later in the book, such user-defined objects extend the capabilities of the language and enable structured programming. Lastly, I presented object prototypes—properties of all instances of a certain object type. Although it should be clear by now, do not worry too much about how to use them, as they will be used in many examples later on in the book. Objects are the backbone of JavaScript. Many elements of the language are actually objects, although other languages do not refer to them as such. The classic example for such objects are arrays, which are discussed in depth in the next chapter.

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Arrays
What is an Array?
Arrays are data structures, and are somewhat more complex than simple variables. You can think about an array as a set of variables with the same name. We use arrays when we want to work with a certain set of related data that are all of the same data type. An example would be a set of students’ ages. These would all be integer values and they would be related. As a programmer, if you want to process an individual item of an array you need to specify the array name and indicate which array element is being referenced. Specific elements are indicated by an index or a subscript.

Some History
The first version of JavaScript, the one supported by Navigator 2.x, did not feature arrays. This was very strange because whether a language is a scripting language like Perl or a programming language like C++, it must have an explicit array type. JavaScript scripters had no choice but to come up with a simple workaround. Everyone used (and a few still use) the following function to create arrays:
function createArray(n, init) { this.length = n for (i = 1 ; i <= n ; i++) { this[i] = init } }

This method is not frequently used any more and is not necessary. However, you may see it in some code so I am showing it to you so that you are not confused if you do.

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Creating Simple Arrays
Arrays in JavaScript are simple built-in objects. You create an array just like an instance of an object, because that is exactly what it is. The formal name of the object is Array—notice the capital “A.” The general syntax is:
var arrayObjectName = new Array() var arrayObjectName = new Array(arrayLength)

arrayObjectName is either the name of a new object, an existing variable, or a property of an existing object. arrayLength is the number of individual elements in the array. In JavaScript specifying the length of the array or the number of elements is not vital. An array is an object like any other object. There is no explicit array type in JavaScript. Here are some arrays:
var day = new Array(31) var month = new Array(12) var year = new Array() // number of years "not known"

All elements of an array are initially null. This is important because such elements do not have any influence on your script. An element with a null value actually does not exist. You can prove this to yourself by running getProps() on a new array:
function getProps(obj, objName) { var result = "" for (var i in obj) { if (typeof obj[i] == "object") result += getProps(obj[i], objName + "[" + i + "]") else result += objName + "[" + i + "] = " + obj[i] + "<BR>" } return result } var ar = new Array(3) document.write(getProps(ar, "ar"))

This script does not generate any output, because if a property of an object has a null value, it does not exist in the computer memory; thus it is not recognized. Strongly typed programming languages require that you specify the number of elements in an array. In such languages all elements of an array must contain values of the same type (int, char, …). An array is an explicit data structure in those languages, so it is not an object as in JavaScript. null values are not given to the elements of an array. From the moment an array is declared, it takes up the required storage area in memory. It does not matter if you initialized its values or not. Theoretically, if you created an array without specifying the number of elements, it would

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be as if you created one with zero elements. Extending an array in such languages is usually not possible; therefore, you must specify the length in all array declarations. The null value is given by default to each element of an array in JavaScript. When the value is null, the element is not stored in the computer’s memory. So creating an array with an infinite number of elements is fine in JavaScript. Keep in mind that elements holding a null value do not take up space in memory, but they are considered elements in the array, so they are added to the array’s length property. With JavaScript, like Perl and unlike C++, it doesn’t matter whether the elements of an array are of the same data type. They can be a mix of numbers, strings, Boolean values, objects, and even arrays (which are actually objects).

Referring to an Array’s Elements
To use an array you must be able to refer to its elements. Arrays in JavaScript are objects. Like all other objects, they have properties and methods:
arrayObjectName[subscript] // ar1[4]

The subscript follows the array name and is enclosed in square brackets. Subscripts are simple integers which start at zero. Here is a simple array:
var ar = new Array(5)

This array has five elements: ar[0], ar[1], ar[2], ar[3], and ar[4]. After you create an array you can increase its length by specifying a value for the highest subscript element. The following code creates an array of length zero, then assigns a null value to element 99. This changes the length of the array to 100.
accounts = new Array() // array of zero elements accounts[99] = null // array of 100 elements

Note that the array does not take up any storage space, even after it is extended. When referring to an element, the subscript can be either a literal (e.g., 3) or a variable (e.g., num = 3). An element of an array can be any valid value. It can be a string, a number, a Boolean value, a null value, or even another object. For example, if you want to create an array in which each element is a student object, you can use the following statements:
function student() { // constructor function // properties not initialized to meaningful value this.name = "" this.age = "" this.grade = "" } var size = 35 // num of students in class var students = new Array(size) // array is defined

At first, the desired size of the array, the number of students in the class, is assigned to the variable size. An array of that size is then created. All elements of the array, from students[0] to students[34], are then defined using the constructor function student(). In this example, all of the elements in the array are of the same type. An array can also have elements of different types as shown in the following code segment.
function student() { // constructor function // properties not initialized to meaningful value this.name = "" this.age = "" this.grade = "" } function teacher(name, age) { this.name = name this.age = age } var size = 35 // num of students in class var students = new Array(size + 1) // array is defined students[0] = new teacher("Kate", 45) for (var i = 1; i < size + 1; i++) { // or i <= size students[i] = new student() } alert(" is the teacher." + students[0].name)

In this script segment an array of size + 1 elements is defined, because the first element, students[0], holds an instance of the teacher object. The most important rule is that the subscript, or index, starts at zero. Although it might seem quite awkward, use this element like all other elements of the array.

Creating Dense Arrays
You can construct a dense array of two or more elements starting with index [0], if you define initial values for all elements. A dense array is one in which each element has a value. They are very popular in many scripting languages, especially in Perl. You can populate an array by specifying the values of its elements:
var bb = "baseball" var sports = new Array("football", bb, "basketball", "soccer")

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You can refer to the elements of this array with the common syntax:
sports[0] == "football" sports[1] == "baseball" var val = 2 sports[val] == "basketball" sports[3] == "soccer"

JavaScript for Navigator still lacks several features related to arrays that are supported by Perl: n You cannot assign a range to an array. n You cannot assign elements of one array’s values to another array (the resulting array is an array slice).

Array Types
JavaScript is a loosely typed language, just like Perl. It is not surprising, therefore, that elements of an array can be of different types. Some elements of a given array can be strings, other can be numbers, Boolean values, and even objects. Basically, there are five types of arrays: n String arrays n Number arrays n Boolean arrays n Object arrays (including null arrays, because null is an object) n Mixed arrays Sometimes you want to know what type of array you are dealing with. JavaScript does not include any tool to facilitate this. However, using a prototype we can add a property to all arrays (remember, arrays are objects by which you can create instances) that will return the type of the array. Here is the desired method:
function getType() { var arrayType = typeof this[0] for (var i = 1; i < this.length; ++i) { if (typeof this[i] != arrayType) { arrayType = "mixed" break } } return arrayType } Array.prototype.getType = getType

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The following script segment is based on the preceding prototype definition:
var ar1 = new Array(3) ar1[0] = "a" ar1[1] = "b"

If you tried out the fifth array, you probably had to wait for a while, because the loop executed 10,000 times! You can use a more efficient function for the same prototype:
function getType() { var arrayType = typeof this[0] for (var i in this) { if (typeof this[i] != arrayType) { arrayType = "mixed" break } } return arrayType } Array.prototype.getType = getType

The improvement in this function is the type of loop used. The for...in construct loops only through the existing properties, or elements, not including the null ones. The function works just like the previous one. At first, the data type of the first element of the array is assigned to arrayType. In every iteration of the loop, if a different type element is found, the value of arrayType is changed to “mixed” because at least two different data types have been found in the array. Once a “mixed” array is detected, the loop is immediately terminated using a break statement. The function returns the value held by arrayType. Sometimes you want to refer to all elements of a uniform-type array. The following method is designed to be a prototype of the Array object, returning an array of element subscripts where the specified value has been found. Here is the method:
function getSubscripts(type) { var ar = new Array() var arSub = 0 for (var i = 0; i < this.length; ++i) { if (typeof this[i] == type) {

Strings in JavaScript are String objects, not arrays. There are many useful methods that operate only on strings, so dealing with strings as arrays of characters, as is often the practice in other languages, is useless and almost impossible. Chapter 14 deals with strings in depth. If you try to print an array, you will see that JavaScript prints the values of all elements in consecutive order, with a delimiting comma in between. You can use a user-defined prototype method to return a string containing all values delimited by a user-provided string:
function getList(str) { var text = "" for (var i = 0; i < this.length – 1; ++i) { text += this[i] + str } text += this[this.length – 1] return text } Array.prototype.getList = getList var ar = new Array(5) ar[1] = 3

Notice that the loop in the method executes until i < this.length – 1. The reason is that the loop concatenates the delimiter (the method’s parameter) after each element of the array. We do not want one placed after the last element, so the last element is concatenated to the accumulated string after the loop terminates.

Array Properties
Although you can add your own properties to the Array object, not all tasks can be achieved by high-level programming via JavaScript. The only built-in property of the Array object is length. When you create an instance of an object (an array), the number of properties (elements) is stored in the length property. You can refer to it as a regular property. We have already seen this property in action earlier in this chapter. Let’s say you want to display some messages, one after the other. You should use an array to store the messages:
var messages = new Array() messages[0] = "message 1" messages[1] = "message 2" messages[2] = "message 3"

You can then use a loop to display the messages successively:
for (var i = 0; i < messages.length; i++) { document.write(messages[i] + "<BR>") }

The array consisting of six elements was reduced to three. The last three values were chopped off. Be extra careful when you reduce the size of an array because shifted-off values are unrecoverable.

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Array Methods
In this section we will deal with many array methods. We will simulate some array functions in Perl using prototype methods. JavaScript features three built-in methods which we shall also discuss: n join() n reverse() n sort() In this section we use the getList prototype method, introduced in the section “Array Types,” to print arrays.

chop()
The chop() method chops off the last character of all strings that are elements of an array. Here is an example:
function chop() { for (var i in this) { if (typeof this[i] == "string") this[i] = this[i].substring(0, this[i].length – 1) } } Array.prototype.chop = chop // EXAMPLE var line = new Array("red", "green", "blue") line.chop() document.write(line.getList(" "))

The script’s output is:
re gree blu

The general syntax is:
arrayInstance.chop()

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The substring() method is explained in Chapter 14, Handling Strings.

grep()
The grep() method evaluates the expression (expr) for each element of the array. It returns another array consisting of those elements for which the expression evaluated to true (the pattern was matched). To understand this method you should know that if string2 is found in string1, string1.indexOf(string2) is not equal to –1. Here is the method:
function grep(str) { var ar = new Array()

An important concept used by this method is short-circuit evaluation. If JavaScript evaluates the first conditional expression (typeof this[i] == "string") to false, the final expression (typeof this[i] == "string" && this[i].indexOf(str) != –1) is obviously false, so the second expression (this[i].indexOf(str) != –1) is not even evaluated. This is critical because the indexOf() method works only with strings, and generates an error otherwise. However, if the current element (this[i]) is not a string, JavaScript does not evaluate the second expression due to short-circuit evaluation.

join()
The join() method is a built-in one in JavaScript. It is equivalent to the same function in Perl. It joins the elements of an array into a single string and separates each element with a given delimiter. This method is exactly like the getList method we created earlier, so we will use it from this point on instead. Its general syntax is:
arrayInstance.join(delimiter)

The delimiter is automatically cast to a string if it is not already one. Here is an example using this method:
var line = new Array("a", "b", "c", "d", "e") document.write(line.join(" : ")) var str = line.join(", ") document.write("<BR>" + str)

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Its output is shown in Figure 11-1:

Figure 11-1. Example output of the join() method.

pop()
The pop() method pops off the last element of an array and returns it. The array size is automatically decreased by one. The general format is:
arrayInstance.pop()

This method is not built in, so we need to define it. The following script defines the method as a prototype of the Array object and demonstrates its use:
function pop() { var lastElement = this[this.length – 1] this.length-return lastElement } Array.prototype.pop = pop

Note: This method was not supported by earlier versions of JavaScript but is now supported.

The reverse() method transposes the elements of the calling array object. If it was descending, now it is ascending, etc. The last element becomes the first one, and vice versa. This is a built-in method. The general syntax is:
arrayInstance.reverse()

shift()
The shift() method is not defined in JavaScript so we will have to create it. It shifts off and returns the first element of an array, decreasing the size of an array by one element. The general format of this method call (after defining its prototype) is:
arrayInstance.shift()

sort()
The sort() method, which sorts the elements of an array, is built into JavaScript. It sorts the elements of an array. It is optional to supply a sorting function. If one is not supplied, the array is sorted lexicographically (comparison order, dictionary order), according to the string conversion of each element. The general syntax of this method is as follows:
arrayInstance.sort(compareFunction)

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If compareFunction is not supplied, elements are sorted by converting them to strings and comparing the strings in lexicographic order. For example, “10” comes before “9” in lexicographic order, but numeric comparison puts 9 before 10. The structure of a comparison function is very specific. First of all, it should have two parameters, one for each element being compared. Secondly, it should return a value. If a and b are the two elements being compared, then: n If compareFunction(a, b) is less than zero (returns a negative number), sort b to a lower index than a. n If compareFunction(a, b) returns zero, leave a and b untouched with respect to each other (if a was before b, it will remain before b, and vice versa).

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If compareFunction(a, b) is positive (greater than zero), sort b to a higher index than a.

The basic form of a comparison function is:
function compare(a, b) { if (a is less than b by some ordering criterion) return –1 if (a is greater than b by the ordering criterion) return 1 // a must be equal to b return 0 }

This method is more complicated than the previous ones. At first, two arrays are declared. Although it is possible to calculate their size, it is completely unnecessary because they will accommodate the proper size as we expand them. The first loop in the function is:
for (i = 0; i < length; ++i) { ar1[i] = this[i + offset] }

This loop assigns all elements that are to be removed to the array ar1, according to the values of the offset and length parameters. The number of times the loop iterates is exactly the value of length, the number of elements specified to be removed. The array ar1 is populated from the beginning with 0. It is assigned the elements of the calling array (this), starting from offset, because only elements following offset (exactly length of them) are to be removed. When the loop terminates, the array ar1 already consists of elements that should be removed from the calling array. Notice that this array is returned at the end of the function. Now take a look at the second loop:
for (i = 0; i < this.length – (offset + length); ++i) { ar2[i] = this[i + offset + length] }

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This loop assigns all elements following the removed ones to the array ar2. The loop terminates after it has completed assigning all these elements. this.length – (offset + length) is equal to the number of elements following the removed one. The subscript of the first element following the removed set is offset + length. Subtracting this number from the full length of the calling array gives the number of elements following the removed set.

The preceding loop assigns all items of list (the third parameter of the method) to the array args. These items are all arguments handed over to the method except for the first two. We use the condition splice.arguments[i + 2] != null to terminate the loop due to some unexpected behavior of the arguments.length property on some platforms. That is, when the function comes across the first null element of the splice.arguments (see Chapter 6, Functions and Variable Scope), it terminates. This action is based on the fact that the method will never be called to replace an element of the calling array by a null one. This concept is explained later in this chapter in depth. The expression i + 2 is used because the needed elements of the arguments array are only the third one (subscript == 2) on. However, the args array created in the function should store the elements starting at subscript 0, so i is used as the subscript of that array. We now have four arrays: n this (the calling array) n args n ar2 n ar1 Keep in mind that the splice() method needs to modify the calling script. It concatenates the beginning of this array (up to the first removed, exclusively) with args and ar2, in this order. The last portion of the script does just that:
j = offset for (i = 0; i { this[j] j+++ } for (i = 0; i { this[j] j++ } this.length = < args.length; ++i) = args[i]

< ar2.length; ++i) = ar2[i]

j

At first, j is set to offset, the first value of the array this that should be modified (all the existing elements before should remain at the beginning of the array, as they were before calling the method). The following loops assign elements from the other arrays, args and ar2, respectively, to the calling array (this). The variable j is important because it holds the subscript of the current element of the calling array throughout both loops. At the end, the length of the calling array (now modified) is set to j, so if there were more elements in the original array, they are chopped off.

The function will not work if you try to remove elements of the array without inserting new ones.

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The split() method does the opposite of join(). It splits up a string (the object) by some delimiter (space by default) and returns an array. To be accurate, this is a method of the String object, not the Array object, but because it is closely related to arrays we chose to discuss it in this section. If the delimiter is not supplied or is not found in the entire string, it returns the string itself. This method is built in so you just have to call it. The general format is:
stringName.split(delimiter)

All of these examples follow the rules explained before, except for the last one, ar4. The statement ar4 = line1.split("") causes an infinite method execution that will eventually crash the browser, or even the operating system, if you are on a 16-bit version of Windows. The reason for this behavior is obvious—JavaScript tries to split the string with an empty string. Everyone knows that an empty string is found an infinite number of times between two characters, and between two empty strings lies another empty string!

unshift()
The unshift() method is actually the opposite of the shift() method. It appends a list of elements to the beginning of an array. Here is the method defined as a prototype of the Array object type, along with an example to demonstrate it:
function unshift() { for (var i = 0; i < unshift.arguments.length; ++i) { if (unshift.arguments[i] == null) break } // i = number of arguments! (remember ++i is executed during last loop) // i holds the number of arguments for (var j = this.length – 1; j >= 0; --j) {

Here is an example of the unshift function:
var line = new Array("ccc", "ddd", "eee") line.unshift("aaa", "bbb") document.write(line.join(" "))

The expected output is shown in Figure 11-8:

The arguments Array
The functionName.arguments array holds the arguments by order that were passed to the function functionName when that function was last called. Here is an example:
function func() { document.write(func.arguments[2]) }

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Figure 11-8. Example output of the unshift() method.

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func(1, 2, 3, 4, 5) document.write("<BR>") func()

The output of this script is:
3 3

The second 3 is not an argument passed to the function during the second call, but an argument passed to the function during the first call. It remains an element in the array provided that no argument of a later call replaces it. You can therefore conclude that the functionName.arguments array has a permanent storage class. Let’s say you want to create a function that accepts string arguments and prints them with a line break after each one. The intuitive function would be:
function printList() { for (var i = 0; i < printList.arguments.length; ++i) { document.write(printList.arguments[i] + "<BR>") } }

You can use this function in the following form:
printList("message 1", "message 2", "message 3", "message 4")

The function now displays the number of arguments according to the functionName.arguments.length property. It also displays the fourth argument, whether or not there are four arguments. Consider the following statements:
printList("message 1", "message 2", "message 3", "message 4") printList("message 5", "message 6", "message 7")

Notice that the function prints the fourth argument passed to the function in each call. However, the second call provides only three arguments! The arguments.length property holds the correct value, 3. The fourth argument is actually a leftover from the first function call. As you can see, the arguments array does not follow the rules of arrays that are instances of the Array object. The following script demonstrates the difference:
var ar = new Array("message 1", "message 2", "message 3") document.write("The array is: " + ar.join(", ") + "<BR>") document.write("There are " + ar.length + " elements.<BR>") document.write("The third element is " + ar[2] + ".<BR>") ar.length = 2 document.write("There are " + ar.length + " elements.<BR>") document.write("The third element is " + ar[2] + ".<BR>")

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The output of this script is:
The array There are The third There are The third is: message 1, message 2, message 3 3 elements. element is message 3. 2 elements. element is null.

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The simple rule regarding arrays is that the last element’s subscript is arrayName.length – 1. When you reduce the size of an array by assigning a smaller number to the length property, all elements that were in the range of the array and are now out of range are lost. If you try referring to them, you see that they are null. The functionName.arguments array is different in that its last element has nothing to do with its length.

Multidimensional Arrays
JavaScript does not feature multidimensional arrays, but they are fairly easy to create via a constructor function. Up to this point we discussed one-dimensional arrays (1D arrays). Such arrays consist of elements gathered in a row-like structure. The basic structure of a one-dimensional array is outlined in the following illustration:
Figure 11-9. A onedimensional array.

Let’s say you want to store a runner’s record times in different years and for different distances. If the array is called curDis, the record time of 1994 could be put in curDis[0], the record time of 1995 in curDis[1], the record time of 1996 in curDis[2], and so forth. However, you probably noticed that we can only store the record time for a single distance, such as a 400-meter run. It would be easy to store the record times of various runs in this format using a table or spreadsheet-like array. Such arrays are known as two-dimensional (2D) arrays. Figure 11-10 outlines the basic structure of a two-dimensional array.

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Figure 11-10. A twodimensional array.

JavaScript does not have a built-in two-dimensional array object type. Take a look at the subscripts of the elements. The two-dimensional array is constructed from horizontal arrays (horizontal in the illustration as well) which form an array of their own. That is, the general structure is a regular array of arrays. The main array in the illustration is the vertical one; its subscripts are specified to the left of the arrows. Note that the dimensions of a multidimensional array do not have to be equal. The standard reference to elements of a two-dimensional array (2DA) is:
ar[1][5] ar[8][0] ar[3][3]

The first number is the subscript of the first dimension, whereas the second one specifies the subscript of the second dimension.

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Creation with a Single Constructor Function
The easiest way to create a two-dimensional array (2DA) is to use a constructor function such as the following:
function Array2D(dim1, dim2) { for (var i = 0; i < dim1; ++i) { this[i] = new Array(dim2) } this.length = new Array(dim1, dim2) }

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The following script demonstrates the creation and usage of an array created by this function:
var ar = new Array2D(4,7) ar[2][1] = 6 ar[0][0] = "Hello" // this is the "first" element ar[3][6] = true // this is the "last" element alert("Length of first dimension is " + ar.length[0]) alert("Length of second dimension is " + ar.length[1]) alert("\"Last\" element: " + ar[ar.length[0] – 1][ar.length[1] – 1])

The messages displayed via alert boxes are:
Length of first dimension is 4 Length of second dimension is 7 "Last" element: true

The constructor function uses a loop. The first array (ar) is not an explicit Array object. It is simply a regular object whose properties are specified using the array notation. The built-in methods of the Array object type do not operate on this array. The loop executes dim1 times, the length of the first dimension. Each property (or element) of the calling object is assigned a “real” array consisting of dim2 elements. Since these are real arrays, you can use the built-in methods and properties of the Array object type. For example, ar[0] is an array of dim2 elements, so you can refer to its elements: ar[0][0], ar[0][1], ar[0][2], etc. The 2DA is actually a one-dimensional array of which all elements are one-dimensional arrays. Therefore, you should be careful not to ruin the structure of the 2DA by assigning values to its main array, such as:
ar[3] = "Do not do this to a 2DA!"

The preceding constructor method also includes a length property that is an array of two elements. The first element holds the number of elements in the main array (the specified length of the first dimension) while the second element holds the specified length of the second dimension. The example demonstrates this best. Keep in mind that these are static properties. Assigning a value to an element of the length property does not affect the array, but can trip you up when you need the correct values regarding the length of the array’s dimensions.

Creation without a Constructor Function
It is possible to create a two-dimensional array without a constructor function. This method is based on creating instances of the Array object:
function addDim2(array1D, dim2) { for (var i = 0; i < array1D.length; ++i) { array1D[i] = new Array(dim2) } return array1D }

At first, the length of the first dimension, 4, is assigned to dim1. The length of the second dimension is then assigned to dim2. A new array, named ar, is created according to the Array object type. Its length is the length of the first dimension of the desired array. The function addDim2 is then called with the array and the length of the second dimension. It returns the final 2DA to the original one-dimensional array, ar. The function is based on a simple loop that assigns an array of dim2 elements to each element of the original one-dimensional array. Since both the first dimension array and the second dimension array are instances of the Array object type, you can refer to the length of each dimension in the following way:
ar.length == length of first dimension ar[0].length == length of second dimension == ar[1].length == …

Note that for the second expression to be true, you must not change the length of the 2DA at any time during the script execution. Otherwise, it does not simulate a 2DA anymore.

Associative Arrays
Associative arrays use strings as subscripts. For example, an element of the array can be:
color["yellow"] = "FFFF00"

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Such arrays group together related data. These arrays are actually objects, except that you use square brackets instead of a dot. Another important difference is that array specification (square brackets) enables you to refer to the value of a variable as the property of method specification rather than the actual literal. For example:
var col = "yellow" color[col] = "FFFF00"

If you replace the array notation with the regular dot specification you receive an error:

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var w = "write" document.w("Hello!")

The dot notation requires the actual literal specified, as it does not evaluate the written value. You can use associative arrays to create nested objects, resembling multidimensional arrays. For example, you can create an associative array where its subscripts are names of students in a class, so each element of the associative array is an object containing fields such as the student’s grade, age, and so on. Note that associative arrays cannot be created as instances of the built-in Array object. You must use a constructor function to create one. Here is an example using students:
function studentClass() { for (var i = 0; i < arguments.length; ++i) { this[studentClass.arguments[i]] = new student() } } function student() { // this.grade = null // this.age = null } var students = new studentClass("Bob", "John", "Frank", "Alfred", "Sheila", "Mary") students["Bob "].grade = 40 students["John "].age = 11 students["Frank "].grade = "N/A" alert(students["Frank "].grade)

Creating an associative array is not difficult. Array elements are created according to the names of the students accepted as parameters by the function studentClass(). The second function is rather strange—it contains no statements. When you create an instance you can use an “empty” constructor function. You may recall from an earlier discussion that objects may be extended by simply assigning values to them. So, the statement this[arguments[i]] = new student() just makes sure each element of the associative array is an object. The global statements later create the properties simply by assigning them values. Another important concept to remember is that associative arrays are not explicit arrays. You can also refer to them using the dot syntax as in:
students.John.age

Suppose you created a database structure listing all the students in a class with their ages and grades. Assume that both fields contain the proper values. You can let the user view these values via a prompt dialog box. For example:
var grade = students[prompt("Enter name:", "John Doe")].grade document.write(grade)

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In this case you must use the array convention because you are prompting the user for the property name. You can use the dot notation only when you know the property name. If you use the dot notation:
students.prompt(...)

JavaScript assumes you are referring to a method of the students object named prompt(), which does not exist at all, of course.

Populating an Associative Array
Creating a dense array is simple when using regular arrays. You simply create the array with the desired values. However, associative arrays require two real values for each element, the key (subscript) and the value. You can create a constructor function to create and populate associative arrays:
function AssociativeArray() { for (var i = 0; i < arguments.length – 1; i += 2) { this[arguments[i]] = arguments[i + 1] } }

Let’s take a look at the constructor function. It accepts the keys of the array elements followed by their corresponding values. Each key must be followed by its own value. The function loops through the arguments and terminates after it has reached the argument before the last one. During each pass through the function, a property of the calling object is created (an “element” in the array lexicon). The key of the property, or element, is the current argument whereas its value is extracted from the following argument. The loop counter is advanced by two after each execution of the block, because each element of the associative array is related to two arguments—its key and its value. Although an associative array is primarily a regular object, this constructor must use square brackets, the array notation, for reference and initialization because the values of the keys (subscripts) are not literals, but rather values stored as parameters.

Notice the use of an empty function to create an object. First, the function splits the string via the regular method to a regular array. It then loops through the array, using the next element as the key of an element in the associative array and its following element as the value of the same element in the associative array. Upon completion, the function returns the associative array. The function is then declared as a prototype of the built-in String object, and applies to all strings. Now take a look at an example:
var str = "a b c d e f" var ar1 = str.associativeSplit(" ") document.write(ar1.a + "<BR>") document.write(ar1.b + "<BR>") document.write(ar1["c"] + "<BR>") document.write(ar1["d"] + "<BR>") // document.write(ar1[e] + "<BR>") document.write(ar1.f + "<BR>")

Note that “associative array” is not a JavaScript term, but rather a regular object. We just prefer to refer to its properties via the array notation.

Summary
In this chapter we learned about arrays in JavaScript. Unlike most languages, an array in JavaScript is a regular object, not an explicit data type of its own. We discussed the Array object, including its properties and methods. Other important concepts brought together in this chapter were constructor functions and prototypes, used mainly to

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simulate array methods featured by Perl. By now, you should have a grasp on arrays and constructor functions, two very important elements of the language. You should also know how to create and use two-dimensional arrays, as well as multidimensional ones, although they are rarely used. In this chapter we also introduced the term “associative arrays.” Such arrays are regular objects with regular properties, but they remind us of associative arrays, widely used in other languages such as Perl. In following chapters we will look further into JavaScript’s object model, while arrays and constructors will serve as the base of some scripts.

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Time and Date in JavaScript
The Date Object
JavaScript provides you with a Date object which allows you to easily work with date and time data and related functions. Both dates and times in JavaScript are derived from the Date object, which includes a rich set of methods related to derivation of dates and times. The Date object behaves much like any other object. To implement the current date or time in your script, you must first create a new instance of the object. You can then extract the desired data from that particular instance. JavaScript handles dates similarly to Java. Many methods are implemented in both languages, resulting in an observable parallelism. All dates are stored as the number of milliseconds since January 1, 1970, 00:00:00.

Creating a Date Instance
The Date object is a single built-in object, by which you can create instances to store encoded data related to the date and time of a certain moment. The Date object is built in, just like the Array object discussed in depth in the previous chapter. It acts as a template when creating instances of the object. The most basic assignment statement regarding the Date object is obviously the one that creates an instance according to the default arguments:
var dateInstance = new Date()

This statement simply assigns an instance of the Date object to the data structure named dateInstance. Take a look at the following script:
var now = new Date() alert(now)

The output is shown in Figure 12-1 (the date and time will be different when you try it):

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Figure 12-1. An alert box showing the current date and time.

Although now is an identifier of an instance of the Date object, it holds a partial string value at its highest level. That is, if you try to print its value, it appears to be a string. However, because it does not belong to the explicit String object, string properties and methods do not apply to it. For example, the following script generates an error because split() is a method of strings only:
var nowDate = new Date() var nowArray = new Array() nowArray = nowDate.split(" ")

Notice in the output that there are two semicolons near the end of the string. You will see later that the time zone is specified between them. If you want to refer to an instance of the Date object as a string, it is safer to convert it to a string explicitly:
var newObj = new Date() var str = newObj.toString()

The toString method simply converts the object to a String object. This is similar to casting in other programming languages.

Parameters of the Date Constructor
Until now we created instances of the Date object without any arguments; that is, we built the instances according to the default arguments. The default is the current date and time on the client’s machine. JavaScript enables you to create Date instances of specific dates, which you can later use with date arithmetic. Here is an example that creates an instance of a date that occurred in the past:
var Xmas95 = new Date("December 25, 1995 13:30:00")

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You can also create an instance according to a set of integers of the following format: year, month, day, hour, minute, seconds Here is an example using this format:
var Xmas95 = new Date(99,11,25,9,30,0)

One important concept is the requirement of year, month, and day specifications in both formats (string and integers). If you omit the hours, minutes, or seconds they are set to zero by default. Nonetheless, omitting any of the first three arguments results in an error and even crashes the browser under some operating systems.

Date Numeric Conventions
Dates in JavaScript, as in Java, use integers to specify values that succeed each other. For example, the first day of a month is 1, the second is 2, the third is 3, and so on. The numbers used are not always so obvious. Most date attributes are actually zero-based; that is, they start at zero. For example, the first minute of an hour is 0, the second one is 1, the third minute is 2, and so on. The following table summarizes the numeric conventions of each attribute of a Date instance:
Table 12-1. Ranges of date attributes. Date Attribute seconds minutes hours day date month year Range 0 - 59 0 - 59 0 - 23 0-6 1 - 31 0 - 11 number of years since 1900 (e.g., 97)

Most importantly, when referring to client-side JavaScript, the date and time refer to the client side. All values are the ones passed to the script by the browser. As you might know, especially if you are a Mac or Windows environment programmer, all applications on your computer have access to the machine clock, including the current time and date. If the system clock is not set to the current time and date, JavaScript will use these incorrect values in the script, possibly surprising the user.

Date Method Categories
JavaScript provides JavaScript programmers with a variety of methods to deal with instances of the Date object. However, extracting and manipulating their data makes them one of the most important elements of the language. The whole bulk of methods may seem dazzling if you approach them at once. Therefore, we have chosen to divide them into four groups, according to their operation:

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n n n n

get methods set methods to methods parse methods

get methods return an integer corresponding to the attribute of the instance you desire. You can “get” the year number, the month number, the hour number, and so on. set methods enable you to modify the value of a certain attribute of an existing instance. These methods accept integer values rather than return them. You actually “set” the values of attributes with these statements. to methods convert the date into a string according to arguments passed over to the method. You can then take advantage of the string format with string methods and properties, such as the method split(). parse methods simply interpret date strings.

get Methods
getYear()
The getYear() method returns the current year stored in an instance of the Date object type. In the past this was a two-digit date that assumed it to be the 20th century. Both the latest versions of Netscape and Internet Explorer, however, return a fourdigit date.
var now = new Date() var year = now.getYear() while (1) { var guessYear = parseInt(prompt("Enter current year:", "")) if (guessYear == year) { alert("That's right!") break } else alert("Wrong answer! Try again...") }

The output of this script, assuming you are able to correctly identify the current year, is presented in Figures 12-2 and 12-3:

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Figure 12-2. A dialog box prompting the user to enter the current year.

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Figure 12-3. The alert box confirming the answer.

The current year (based on the system clock) is extracted from the instance now, created according to default arguments (current time and date). A loop without a terminating condition is executed next. The user is asked to enter the current year, according to his or her knowledge. Another if statement checks whether the user entered the correct year. If so, the proper message is displayed, and the loop is broken up with a break statement. Otherwise, a message informs the user that his or her input was incorrect, and the loop iterates once more. The problem with getYear is that Netscape still returns the years since 1900 (thus 2001 is returned as 101), whereas Internet Explorer simply returns the four-digit date (making 2001 return as simply “2001”). The newer command getFullYear will get a simple four-digit year in both browsers.

The current month is extracted from the now instance which holds the attributes of the current time (after the statement has executed). A static array is then created to hold all the names of the month (as strings), matching each name to its corresponding number in JavaScript, starting at zero. This obviously fits the default array structure, featuring the first index as zero. Therefore, no math needs to be done, and the current month, by name, is used to construct a message, obviously a string. The message is then printed as plain HTML.

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getDate()

Warning: The getMonth attribute returns an incorrect value (off by one) on some older versions of Navigator. (MSIE works fine.)

This script segment combines all methods learned up to now to display a nicely formatted date. At first, all needed attributes of the previously created instance of the Date object are received and assigned to their corresponding variables (e.g., year, month, date). The last digit of the date (1-31) is then assigned to the variable lastDigit. According to the value of lastDigit, the proper suffix is assigned to suf via a nested if-else statement. For the digit 1, the suffix is “st” (1st); for the digit 2, the suffix is “nd” (2nd); for the digit 3, it is “rd” (3rd); for all other digits, it is “th” (5th, 6th, ...). Note that the last else statement associates the “th” suffix with all digits other than 1, 2, and 3. This suffix even applies to digits ending with a 0. An array of month names is created as before. The current date is then combined with its suffix to create a string such as “27th.” This string, along with all the other desired values, is used to build a complete date format, such as “April 24th, 1997.” The full string is printed.

getDay()
This method returns the day of the week as an integer from 0 to 6. The day of the week is calculated according to the other attributes, so this method does not have a corresponding setDay() method. Here is an example:
ar = new Array(7) ar[0] = "Sunday" ar[1] = "Monday" ar[2] = "Tuesday" ar[3] = "Wednesday" ar[4] = "Thursday"

This script segment prints a short greeting according to the time of the day. For example, if it is between 12:00 (noon) and 16:00 (4 P.M.), it prints “Good afternoon!” It is based on a nested if-else construct.

getMinutes()
The getMinutes() method returns the minute attribute of a Date instance. The integer returned is always from 0 to 59. Here is a short example to demonstrate the method:
var now = new Date() var minute = now.getMinutes() var hour = now.getHours() var text = "Don't you have an appointment for " + (hour + 1) text += ":00 ?" if (minute > 49) document.write(text)

At first, the message containing the nearest hour is built. For example, if it is currently 15:55, the message is built with the nearest hour, 16:00. The message is printed if the current time is less than ten minutes from the next hour. Note that if it is 23:59, the hour is presented as 24:00, not 00:00.

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getSeconds()
This method returns the seconds of a given Date instance, from 0 to 59. You can use it in much the same way as you use the getMinutes() method.

getTimezoneOffset()
This method returns the time zone offset in minutes for the current locale. The time zone offset is the difference between local time and Greenwich Mean Time (GMT). Daylight savings time prevents this value from being a constant. The returned value is an integer representing the difference in minutes. The following script shows how to use the user’s time zone offset to figure out where he or she lives:
if (confirm("Are you in the United States?")) { var now = new Date() var curOffset = now.getTimezoneOffset() curOffset /= 60 // convert from minutes to hours var zone = "" var prep = "" if (curOffset == 8) { zone = "west coast" prep = "on" } else if (curOffset == 7) { zone = "mid - west" prep = "in" } else if (curOffset == 6) { zone = "mid - east" prep = "in" } else { zone = "east coast" prep = "on" } alert("I think you live " + prep + " the " + zone + "!") } else alert("Sorry, this script is intended for U.S. residents only")

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The script starts by asking the user if he or she lives in the United States. If not, a message is displayed. Otherwise, the following command block is executed. The area in the United States is determined according to the difference in hours between the local time zone and the GMT. The preceding preposition (e.g., “in”, “on”) is

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determined as well in the if-else construct. JavaScript then builds an appropriate message based on the current location of the user as well as the proper preposition.

getTime()
The getTime() method returns the number of milliseconds since January 1, 1970 00:00:00.

The set Methods
setYear()
This method sets the year attribute of a given Date instance. The following example computes the day of the current date last year:
var now = new Date() var year = now.getYear() now.setYear(year – 1) ar = new Array(7) ar[0] = "Sunday" ar[1] = "Monday" ar[2] = "Tuesday" ar[3] = "Wednesday" ar[4] = "Thursday" ar[5] = "Friday" ar[6] = "Saturday" document.write("Last year, the current day was " + ar[now.getDay()])

First, an instance of the current date is created, and the current year is assigned to the variable year. The year attribute of the instance now is set to one year behind. The day attribute is then extracted from the modified instance, and a message is built based on that day, transformed to a string (via the array).

setMonth()
The setMonth() method sets the month attribute of a given instance of the Date object. The following script sets the month attribute of the current date to May:
var now = new Date() now.setMonth(4)

setDate()
This method sets the date attribute of a given instance of the Date object. The following script prints the day on which the first day of the month occurred:
var now = new Date() now.setDate(1) ar = new Array(7) ar[0] = "Sunday" ar[1] = "Monday"

to Methods
toGMTString()
This method converts a date to a string, using the Internet GMT conventions. The conversion is done according to the operating system’s time zone offset and returns a string value that is similar to the following form:
Tue, 30 Jul 2001 01:03:46 GMT

The exact format depends on the platform. Here is a simple example:
var now = new Date() var ar1 = now.toGMTString().split(" ") document.write("The current time in Greenwich is " + ar1[4])

A sample output of this script segment is:
The current time in Greenwich is 01:08:21

toLocaleString()
This method returns the date in the form of a string, using the current locale’s conventions. If you are trying to pass a date using toLocaleString, be aware that different locales assemble the string in different ways. Using methods such as getHours, getMinutes, and getSeconds will give more portable results. The following example demonstrates this function:

parse Methods
parse()
The parse() method accepts a date string in the IETF standard and converts it to the number of milliseconds since January 1, 1970 00:00:00. The IETF standard date representation is: DayAbb, date MonthAbb year HH:MM:SS TimeZoneAbb An example for this standard is “Mon, 25 Dec 2001 13:30:00 GMT.” This method also understands the continental U.S. time zone abbreviations such as PST (Pacific Standard Time) and EST (Eastern Standard Time). However, time zones outside the United States (and their equivalent in Canada, for instance) do not have a standard abbreviation accepted by JavaScript. For such time zones the offset must be specified; that is, you must specify the difference in hours and minutes between the local time zone and Greenwich Mean Time. For example, in “Mon, 25 Dec 2001 13:30:00 GMT+0430,” GMT+0430 is shorthand for 4 hours, 30 minutes west of the Greenwich meridian. If you do not specify a time zone, the local time zone is assumed according to the settings of the clock in the operating system. If your time zone is not set correctly, you should change it in the control panel, both on Macs and Windowsbased machines. GMT is also known as Universal Coordinate Time, or UTC. The parse method is a static one. It does not belong to a specific instance of the Date object, but to the object type itself. Therefore, it is always used as Date.parse(). Here is an example for this method:
var aDate = "Aug 27 2001" var birthday = new Date() birthday.setTime(Date.parse(aDate))

UTC()
The UTC() method takes a comma-delimited list and returns the number of milliseconds since January 1, 1970 00:00:00, Greenwich Mean Time (GMT, UTC). This is also a static method, so it is called along with the general Date object. You cannot use this method to refer to a specific date in the local time zone, because it constantly refers to the Universal Time Coordinate (GMT, UTC). For example, the following statement

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creates a Date object using GMT instead of local time, as it would if the method was not used:
gmtDate = new Date(Date.UTC(99, 11, 1, 0, 0, 0))

The general syntax of the method is:
Date.UTC(year, month, day [, hrs] [, min] [, sec])

All attributes should be specified as integers.

Time-Related Methods of Other Objects
setTimeout()
The setTimeout() method evaluates an expression after a specified number of milliseconds have elapsed. Its general syntax is:
timeoutID = setTimeout(expression, msec)

timeoutID is an identifier used to identify the current timeout. expression is a string expression or property of an existing object. It is normally a simple statement that is to be executed after the specified time has ticked off. msec is a numeric value, a numeric string, or a property of an existing object in millisecond units. The setTimeout() method evaluates an expression after a specified amount of time. Take a look at the following example:
<HTML> <HEAD> <TITLE>setTimeout() method</TITLE> <SCRIPT LANGUAGE="JavaScript"> function displayAlert() { alert("5 seconds have elapsed since the button was clicked.") } </SCRIPT> </HEAD> <BODY> <FORM> Click the button on the left for a reminder in 5 seconds; click the button on the right to cancel the reminder before it is displayed. <P> <INPUT TYPE="button" VALUE="5-second reminder" NAME="remind_button" onClick="timerID = setTimeout('displayAlert()',5000)"> </FORM> </BODY> </HTML>

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When you click the button, the event handler’s script sets a timeout. The timeout specifies that after 5,000 milliseconds, or five seconds, the function displayAlert() is called. Therefore, five seconds after you click the button an alert box is displayed. This method does not repeatedly execute the specified statement. That is, it does not execute it every five seconds. When the time limit specified has ticked down, the statement is executed and the timeout does not exist anymore. SetTimeout() is a method of the window or frame object, depending on the basic structure of the HTML document it is used in. It is common to use the setTimeout() method for creating a pause between two consecutive calls to a user-defined recursive function. Take a look at the following script:
function alertNumbers(num) { if (num > 10) return alert(num) val = ++num timerID = setTimeout("alertNumbers(val)", 3000) } alertNumbers(0)

This script segment pops up an alert box every three seconds. The displayed message is a number. The first alert box displays the number 0. After three seconds another one displays the number 1. This process continues until the number 10. If you attempt to print the number to the document rather than displaying it in a window (box), an error is generated. The reason for such an error is that by writing to the document after a delay you are trying to change the layout, which has been completed. Another important point is that if the expression provided to the setTimeout() method is a function call, as in this example, and the function requires an argument, then it must be a global variable. Local variables do not work, because setTimeout() is a method of a frame or the window object (window is the default value if no object is specified). Nonetheless, you can use a literal as the argument. Bear in mind that setTimeout() requires a string-encapsulated expression. You can embed a local variable in this expression as follows:
var cmd = "foo(" + num + ")" timerID = setTimeout(cmd, 2000) // or any other time

clearTimeout()
This method cancels a timeout that was set with the setTimeout() method. It is also a method of the frame or window object, so it is discussed later in detail. At this point, it is important that you know how to use it to cancel a timeout. Its general syntax is:
clearTimeout(timeoutID)

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setTimeout() method, because it actually identifies the timeout’s settings according to it. The setTimeout() method sets a timeout; that is, it executes a statement after a specified amount of time. If you want to cancel the time “bomb” during its ticking, you clear it via this method. If you want to change the amount of time set by the setTimeout() method, you must clear it and then set a new timeout. Here is the previous example enriched by the clearTimeout() method:
<HTML> <HEAD> <TITLE>setTimeout() and clearTimeout() methods</TITLE> <SCRIPT LANGUAGE="JavaScript"> function displayAlert() { alert("5 seconds have elapsed since the button was clicked.") } </SCRIPT> </HEAD> <BODY> <FORM> Click the button on the left for a reminder in 5 seconds; click the button on the right to cancel the reminder before it is displayed. <P> <INPUT TYPE="button" VALUE="5-second reminder" NAME="remind_button" onClick="timerID=setTimeout('displayAlert()',5000)"> <INPUT TYPE="button" VALUE="Clear the 5-second reminder" NAME="remind_disable_button" onClick="clearTimeout(timerID)"> </FORM> </BODY> </HTML>

Time and Date Examples
Times and dates are widely used in scripts to achieve many goals and to create various effects. In this section, we shall introduce a few useful scripts that demonstrate the concepts learned in this chapter.

A Simple Digital Clock
The time and date methods are useful for computing time values in JavaScript scripts. The following example shows how to use such values to create an attractive graphical clock on an HTML page:
<HTML> <HEAD> <TITLE> JavaScript clock

Example 12-1 (ex12-1.htm). A simple graphical clock based on time and date methods.

The output of this script is shown in Figure 12-6:

Figure 12-6. A graphical clock displayed using time and date methods.

The first part of the script is built of comments including a copyright notice, the needed images, and some instructions. It is important to add these comments to every public-domain script because it is often difficult to guess what a script does and what additional objects are needed, such as images. The function getPath() accepts the URL of the current document. This includes all portions of the URL, including “http://…” and the name of the file. The function returns the URL up to but not including the filename, and including the last slash. For example, consider the following URL: http://www.wordware.com/index.html Upon acceptance of this string, the function would return:

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http://www.wordware.com/ The first function in the script, setClock(), actually creates the clock, from top to bottom. Notice that each portion of the function is explained by a comment. Take a look at the following statements, taken directly from the function:
var openImage = "<IMG SRC=\"" + getPath(location.href) + "dg" var closeImage = ".gif\" HEIGHT=21 WIDTH=16>"

In this script segment, two constant-like variables are declared and initialized a meaningful value. The first is assigned the opening structure of an <IMG> tag in HTML. Note that location.href is the current URL of the document. Notice also the use of escape sequences (\”). The value assigned to closeImage is independent of local influences such as the URL of the document. It simply assigns the value .gif\" HEIGHT=21 WIDTH=16>. The HEIGHT and WIDTH attributes are based on the actual height and width of the digit images used to display the time. Here is the next portion of the script:
var var var now var now = new Date() hour = now.getHours() minute = now.getMinutes() = null ampm = ""

This section has two important tasks: n It assigns the local hour to hour. n It assigns the minute attribute of the current time to minute. If you do not remember how, read the beginning of this chapter again. The following script segment simply modifies the value of hour according to the regular conventions used in the United States and other countries using 12-hour clock systems. First of all, noon is considered P.M. Furthermore, midnight is written 12:00, not 0:00. Take a look at the following statement:
if (minute < 10) minute = "0" + minute // do not parse this number!

This statement makes sure that the minute attribute holds a two-digit number. That is, if it is originally a one-digit number, a leading “0” digit is placed. Notice that this digit is actually a string. Attempting to parse this string with a function such as parseInt() would convert it to a numeric type, causing its value to change, because it is written in octal notation (leading 0). It must keep the string value throughout the entire script. The following two statements in the script cast the value of minute and the value of hour to strings by concatenating an empty string to them. This is important, because string properties, which can only be used on strings, are used later on in the script. The following statement is a loop:

The loop executes hour.length times. That is, if hour is a two-digit number, the loop executes twice, once for each digit. During each execution, an image tag corresponding to the current digit in the string, the value of hour, is being concatenated to text. For example, if the value of hour is 12, the loop’s command block executes twice. During the first execution, the following string is assigned to text:
text += '<IMG SRC="http://www.netscent.com/dg1.gif" HEIGHT=21 WIDTH=16>'

During the second pass through the loop, this equivalent statement is executed:
text += '<IMG SRC="http://www.netscent.com/dg2.gif" HEIGHT=21 WIDTH=16>'

If the value of hour is a one-character string, the loop obviously executes only once. The following statement in the script is:
text += openImage + "c.gif\" HEIGHT=21 WIDTH=9>"

This statement simply assigns the tag associated with the colon image. Notice that closeImage is not concatenated in this statement because this image’s WIDTH attribute is different from the other images. The next loop is exactly like the one described earlier associated with the variable hour. It only differs in that it relates to the variable minute. The AM or PM image tag is assigned to text, according to the value of ampm. The final statement inside the function instructs JavaScript to return the value of text, the accumulative string of all the HTML tags needed to print the clock. The returned value is printed to the document by a global statement— document.write(text).

Digital Date
This example is similar to the previous one, except that it displays a set of images associated with the current date, rather than the time. It is based on the same functions, so we shall spare the explanation. Refer to the comments inside the script for some short explanations. Here is the script:
<HTML> <HEAD> <TITLE> JavaScript date </TITLE> </HEAD> <BODY> <!-- JavaScript immediate script --> <SCRIPT LANGUAGE="JavaScript"> <!-// image files needed:

Let’s follow the script step by step, explaining the task of every function. getTime() This function simply returns a string representing the current local time in the following format: hours:minutes AM/PM Note that there are no spaces between any characters. The function is based on the same algorithm as the first part of setClock() in Example 12-1. Refer to the explanation regarding that example for further insights. leapYear(year) This function returns true if the current year is a leap year; otherwise, it returns false. The basic rule used for the decision is that a leap year occurs every four years, in the same year of the summer Olympic games. More exactly, if the year is divisible by 4, it is a leap year. Therefore, the modulo operator suits the case perfectly. If year % 4 is zero, the year is divisible by 4, meaning the current year is a leap year. Otherwise, the year is not divisible by 4, so false is returned. An obvious call to this function is:
if (leapYear(current year)) // is a leap year else // is not a leap year

Another possibility is to use the returned value in a conditional statement, or operation (?:). Note that the parameter of the function must accept an integer value, which is reasonable when computing years. getDays(month, year) This function accepts two arguments: a month and a year. An array of 12 elements is then created. The array is an instance of the built-in Array object. Therefore, the keyword new is used. Each element of the array represents the number of days in its corresponding month. ar[0] holds the number of days in January (31); ar[11] holds the number of days in December. The array is simply assigned the proper data, according to the constant number of days in each month. However, the number of days in

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February is not constant. In leap years there are 29 days in February, whereas in all other years there are 28 days. The function leapYear() is used to decide if the specified year is a leap year. This situation is a typical one for a conditional operator, because one of two values is to be assigned to a variable depending on the value of the condition (the Boolean value returned by the function leapYear()). Notice the extensive use of comments to help you understand the script. The value returned by the function is equal to the number of days in the month passed over to the function upon calling. For example, if the value of month is 0 (as passed to the function), the value ar[0] == 31 is returned by the function. Note that both arguments must be integers. The month must be specified as an integer between 0 and 11, 0 representing January and 11 representing December. getMonthName(month) This function accepts the integer value of a certain month (0 for January, 11 for December) and returns the full name of the function, obviously in the form of a string. This function, like its preceding one, uses an instance of the Array object to store constant values. The name of the desired month is retrieved from the array by its index (subscript). setCal() At first, the function creates a new instance of the Date object, holding the attributes of the current local time. The current year is assigned to year via the method getYear(), and the current month is assigned to month via the method getMonth(). The name of the month, returned by getMonthName(), is assigned to monthName. After the current date is assigned to date, the instance now is assigned null, a good JavaScript programming practice. The next statement of the function is:
var firstDayInstance = new Date(year, month, 1)

It creates a new instance of the Date object; this time it is for the first day of the current month. Therefore, the value 1 is used to specify the date. This obviously influences the day of the week on which the date occurred. This value is assigned to firstDay in the following statement. The instance firstDayInstance is then assigned null. This script segment computes the day of the week (Sunday, Monday, Tuesday, etc.) on which the month started. Another possible way to achieve this goal is to create an instance of the current date as usual:
var firstDayInstance = new Date() // not first day yet!

You then need to set the date to 1, via the setDate() method. You should use the following statement to do so:
firstDayInstance.setDate(1)

The next portion of the function consists of only one statement. It assigns days the number of days in the current month.

The arguments are: n Integer value of the first day of the month + 1; that is, 1 for Sunday, 2 for Monday, 3 for Tuesday n The number of days in the specified month n The specified date n The name of the specified month (e.g., “January,” “February,” “March”) n The specified year, as a four-digit integer (e.g., 1997, 1998) drawCal(firstDay, lastDate, date, monthName, year) This function’s main task is to print the calendar table. Before it does so, the HTML structure of the table must be constructed. The first part of the function assigns values to attributes associated with the final format of the table. Such attributes are the size of cells, font colors, and more. Here is the full list, including the variable names and their roles:
Table 12-2. Variables in drawCal() and their role in the final format of the calendar. Variable headerHeight Role The height of the table header’s cell. The header cell is the cell containing the name of the month and the year in a large font. The height is specified in pixels. The table’s border. You should already know that HTML tables have a BORDER attribute. This attribute specifies the three-dimensional height of the border. The width of the border. A table border’s width can also be set in HTML. This value is the distance between the inner line of the border and its outer line. The color of the header’s font. This is the color of the font in the largest cell of the table at the top of the calendar. The size of the header’s font (see role of headerHeight). The width of the table’s columns. This is actually the width of each cell, or the width of the widest cell in each column. The height of the cell containing the names of the days (“Sunday,” “Monday,” “Tuesday,” etc.) The color of the font representing the days of the week. The height of all the regular cells in the table containing the dates of the month. The color specifying the current date in the calendar. The color of the font used with the current time, located in the cell of the current date.

These are the tags used to create each of the cells containing the day names. For example, the syntax for “Sunday” using the default values of the variables is:
<TD WIDTH=60 HEIGHT=25><FONT COLOR="midnightblue"></FONT></TD>

In the first segment, a regular array is created. It is then assigned the abbreviated names of the days. This array enables us to refer to each name via a number. The following portion, where a cell is created for each day, takes advantage of this referencing method. A new day is printed on every iteration of the loop. Note that the tags associated with the beginning and the end of the table’s current row are not located inside the loop. A new row with all day names is started before the loop. The current row is terminated after the tags related to the cell of “Sat” are assigned to text. The following portion of the function is:
var digit = 1 var curCell = 1

You will see the role of these variables later in the function. By now, all tags associated with the table’s header and the column headers have been assigned to the accumulative string variable text. The remaining part of the function assigns the tags associated with all table’s cells. As you know, the calendar is a rectangular table. Therefore, we prefer to use a nested loop structure to refer to its cells. If you want to practice your skills, try replacing this structure with a single loop, using the modulo operator to compute the location of new table rows.

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The more difficult part of the loop is its terminating condition. Here it is again:
row <= Math.ceil((lastDate + firstDay – 1) / 7)

The Math object and its ceil() method are explained in detail in Chapter 13, JavaScript Math. For now, you should just know that Math.ceil(num) evaluates to the nearest integer to num that is equal to or greater than num (rounding up). Here are some examples:
Math.ceil(15.15) == 16 Math.ceil(16) == 16 Math.ceil(16.0001) == 17

You may recall from the previous function, setCal(), that the value passed over to the parameter firstDay is from 1 to 7, not from 0 to 6. Therefore 1 is subtracted from firstDay in this expression. Math.ceil((lastDate + firstDay – 1) / 7) represents the minimum number of rows needed in the calendar, or table. The number of cells in the calendar (not including the main header, column headers, and cells after the last day in the month) is lastDate + firstDay – 1, because lastDate is equal to the number of days in the month, and firstDay – 1 is equal to the number of cells before the first date. The value is divided by 7, to get the exact minimum number of rows needed. However, the loop must execute a whole number of times. Therefore, the Math.ceil() method is needed. Other, more simple calendars just use five rows for every month, no matter what. However, this simple rule of five rows per month fails when (a) the first day of a non-leap year February occurs on Sunday, meaning only four lines are needed, and (b) the first day of a 31-day month is on Friday or Saturday, meaning six rows are needed. Although these situations seldom occur, you must take them into account. If the row computation is replaced by a simple 5 in this script, a month such as February 1987 does not appear properly. The nested loop is not nearly as difficult, because it always executes seven times, once for each day of the week. Throughout the entire loop construct, including inner and outer loops, digit holds the current cell to be created. The variable curCell holds the accumulative number of cells in the table created (assigned) thus far. This variable is only needed until the cell of the first day of the month is created, and is not handled afterwards; that is, after the first day of the month, it is not incremented anymore. The <TD> and </TD> tags are used to create blank cells, used as placeholders. These are only needed before the first day of the month, because the loop is terminated via a break statement after the last day of the month, and the remaining place in the line is filled in the same way as blank cells (placeholders). Each execution of the inner loop creates the current cell of the table. There are basically two types of cells: n A cell representing the current day, which uses a special font color and displays the time inside the cell n All other cells The HTML tags used are obvious and are not dealt with in depth here.

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There are two statements outside the inner loop but inside the containment loop. The first one creates a new table row and the second one ends the current row. Each execution of the inner loop (seven executions of its command block) is responsible for the creation of an entire row, or a partial one if it is the last row and the last day of the month is encountered before the last cell of the row—in this case the loop is terminated via a break statement. The term “create” refers to the concatenation of the proper strings and values and assignment to the variable text. The last, but definitely not least, statement of the function is the one that actually prints the table to the HTML document. Up to that statement, the HTML document was stored as a string in the variable text.

Random Quotes
The finale for this chapter is a simple, yet interesting script to display a different message each time the page is loaded. Here is the script:
<HTML> <HEAD> <TITLE>Random quote</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function getMessage() { // create array of Murphy's laws var ar = new Array(20) ar[0] = "Nothing is as easy as it looks." ar[1] = "Everything takes longer than you think." ar[2] = "Anything that can go wrong will go wrong." ar[3] = "If there is a possibility of several things going wrong, the one that will cause the most damage will be the one to go wrong." ar[4] = "If there is a worse time for something to go wrong, it will happen then." ar[5] = "If anything simply cannot go wrong, it will anyway." ar[6] = "If you perceive that there are four possible ways in which a procedure can go wrong, and circumvent these, then a fifth way, unprepared for, will promptly develop." ar[7] = "Left to themselves, things tend to go from bad to worse." ar[8] = "If everything seems to be going well, you have obviously overlooked something." ar[9] = "Nature always sides with the hidden flaw." ar[10] = "Mother nature is a bitch." ar[11] = "It is impossible to make anything foolproof because fools are so ingenious." ar[12] = "Whenever you set out to do something, something else must be done first." ar[13] = "Every solution breeds new problems." ar[14] = "Trust everybody ... then cut the cards." ar[15] = "Two wrongs are only the beginning." ar[16] = "If at first you don't succeed, destroy all evidence that you tried." ar[17] = "To succeed in politics, it is often necessary to rise above your principles." ar[18] = "Exceptions prove the rule ... and wreck the budget." ar[19] = "Success always occurs in private, and failure in full view."

Example 12-4 (ex12-4.htm). A script to display a random message each time the page is loaded.

The first statement in the function creates an array, an instance of the built-in Array object. The array includes 20 elements, starting from ar[0] and ending with ar[19]. Each element is assigned a string, or to be exact, a Murphy’s law. An instance of the Date object, now, is then created. The number of seconds in the current time is retrieved from now via the getSeconds() method. As you know, the value of sec is an integer from 0 to 59, with a random possibility for each. In total there are 60 consecutive integers. Due to this fact, the expression sec % 20 returns an integer from 0 to 19, with an equal chance for each, because 60 is divisible by 20 (60 / 20 = 3!). The ability to create a random number from 0 to 19 using this technique enables us to randomly choose a Murphy’s law from the array. The selected Murphy’s law is displayed by an alert box. The most important part of the script is the use of an event handler to respond to the load event—onLoad. When the event handler is triggered (when the page has completed loading), the function getMessage() is called to display an alert message as described earlier. Also notice the use of an escape sequence, the carriage return character (\r).

Summary
In this chapter we discussed date- and time-related concepts in JavaScript. Such concepts are based on the Date object. We learned about this built-in object, the only one in JavaScript without any properties. After a thorough look at the object, we saw how its instances can be created and used in scripts. By now, you should have enough tools to create interesting scripts of various types. As we look further into the language in the following chapters, you will be able to grasp the power of JavaScript to enhance HTML documents. In this chapter you have seen that without much effort, you can add a graphical digital clock to any page. This may seem amazing, but the best is yet to come, including animated clocks and more.

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JavaScript Math
Math in JavaScript
One of the primary capabilities that computers are renowned for is their ability to perform math very accurately and very quickly. All programming languages include features, constants, functions, and other math elements that allow the programmer to perform mathematics. Likewise, JavaScript includes many features related to math, enabling JavaScript programmers to invoke numeric calculations in scripts. Math in JavaScript is based on two general elements of the language: n Mathematical operators n The built-in Math object Mathematical operators are common to almost every language (+, –, *, /, %), and were covered earlier in this book in Chapters 4 and 7. However, I have not yet introduced you to the Math object. In this chapter I will familiarize you with this object, including its properties and methods. We shall also take a look at some possible uses of this object. If there are any mathematical operations covered that you are not familiar with, don’t be too concerned since that simply means you probably won’t be needing to use that level of mathematics in your scripts.

The Math Object
JavaScript’s Math object provides many arithmetic and trigonometric functions. Most of these are functions you probably already know. These functions expand the mathematical ability of JavaScript beyond the basic arithmetic operators. You already know how to create instances of dynamic objects, such as Date. Instances are created according to dynamic objects. Some objects in JavaScript are static ones. That is, they act like a single instance. You can refer directly to the object’s methods and properties. This is different from dynamic objects such as Date. Recall that to retrieve the current year, you must first create an instance of the object and then refer to the instance’s methods.

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To access elements of the Math object, you do not need to create an instance. You access them via messages sent directly by the Math object itself. For example, the PI constant is a property of the Math object, and can be accessed via the following syntax:
var pi = Math.PI

The reason that the Math object is static is that mathematical constants and functions never change. For example, the value of pi will not change, nor will any trigonometric function. The Math object implementation is inspired by the Math class in Java. Despite this parallelism, you will find that JavaScript offers fewer methods.

Constants
Constants are defined with the full precision of floating-point numbers in JavaScript. In this section we shall outline these properties for your reference and understanding. Notice that all properties are specified with capital letters, although this convention is not common in the mathematical community. All properties refer to well-known constants (read your math books if you don’t remember!); that is, they do not change. Therefore, these properties are read-only properties, and accessing them for the purpose of modification results in a JavaScript error.

E
A very important constant in mathematics is Euler’s constant. Its approximate value is 2.718281828459045. It is rounded off to 15 digits after the decimal point. For your reference, the equation is: ei? + 1 = 0 In JavaScript you refer to it via a capital “E”; that is, Math.E.

þ
LN2

Note:

In mathematics it is usually referred to with a lowercase “e”.

Another constant featured as a property of the Math object is the natural logarithm of 2. Its approximate value is 0.6931471805599453. A defining equation is: eLN2 = 2 JavaScript refers to this number as LN2. Because it is a property of the Math object, you should specify them together as in Math.LN2.

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JavaScript’s Math object is a static one. Its properties are actually basic constants, such as pi ( ) and the square root of 2. Its methods are mathematical functions, such as pow() (power), sin(), cos(), and others. These methods and properties are encapsulated in an object because objects are meant to be entities constructed of related data and functions. The requirements from mathematical functions and constants led JavaScript’s developers to create a built-in object for mathematics—Math.

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You can use the pow method to assure that the preceding equation is true:
document.write(Math.pow(Math.E, Math.LN2))

Because both Euler’s constant and the natural logarithm of 2 are approximate, the output of this statement is also approximate:
1.9999999999999998

LN10
The natural logarithm of 10 is also featured as a property of the static Math object. Its value, as stored in its corresponding property, is 2.302585092994046. Once again, you can understand this value via an equation: eLN10 = 10 In JavaScript this value is referred to as Math.LN10. Once more, here is a JavaScript statement to define the natural logarithm of 10:
document.write(Math.pow(Math.E, Math.LN10))

Since both Euler’s constant and the natural logarithm of 10 are approximate, the output of this statement is also approximate:
10.000000000000002

LOG2E
Another important constant in the math arena is the base-2 logarithm of Euler’s constant. Its approximate value is 1.4426950408889634. In math that is: 2LOG2E = e As you can see, you refer to this constant in JavaScript as Math.LOG2E. Here is a simple statement to confirm the value:
document.write(Math.pow(2, Math.LOG2E) – Math.E)

This time the output is apparently exact:
0

LOG10E
The base-10 logarithm is also widely used in complex mathematical calculations. Its value in JavaScript is approximately 0.4342944819032518. The following equation demonstrates the definition of the constant: 10LOG10E = e As you can see, the equation is built according to one of the basic logarithm rules. In JavaScript, log base-10 of Euler’s constant is a property of the Math object: Math.LOG10E.

PI
Probably the most known value among all constants featured by JavaScript is PI. Its approximate value, as enabled by the precision limits of real numbers in JavaScript, is 3.141592653589793. As before, here is an equation to demonstrate this value: = 4(1 – 1/3 + 1/5 – 1/7 + 1/9 – …) As you might expect, you refer to pi in JavaScript as Math.PI. For example, to obtain the circumference of a circle you can use the following function:
function circumference(diameter) { if (typeof diameter == "number" && diameter >= 0) return Math.PI * diameter }

Note that the function does not return a value if the diameter is not a number or is not positive.

SQRT1_2
The square root of 0.5, as stored in JavaScript’s object model, is 0.7071067811865476. You can also reach this value by calculating the square root of 0.5 (using the sqrt() method of the Math object), but accessing an existing property is somewhat faster and more obvious than calculating it via an execution of a method. You refer to this property as Math.SQRT1_2. The reason an underscore is used for the property name is that the name of a property must follow the identifier rules which allow only letters, numbers, and underscores in the middle of a name. Here is an obvious statement to confirm that the value is correct:
document.write(Math.pow(Math.SQRT1_2, 2))

The not-so-obvious output is:
0.5000000000000001

SQRT2
The square root of 2 is also a well-known constant. Its approximate value is 1.4142135623730951. You refer to it as Math.SQRT2. You can use the following statement to ensure the value:
document.write(Math.pow(Math.SQRT2, 2))

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As you could expect, the result is not an exact one:
2.0000000000000004

Math Methods
Constant values make up only a fraction of the entire strength of mathematical implementation in JavaScript. To harness the power of the Math object you have to familiarize yourself with the set of methods available. JavaScript tends to organize functions and values in object structures to enable easy reference and simple understanding. For this reason, all functions related to math are implemented as methods of the Math object. The methods of the Math object can be divided into two categories, each one related to a different branch of mathematics: n Arithmetic methods n Trigonometric methods All methods of the Math object are specified in lowercase letters, as opposed to constants which are properties of this object and are specified in uppercase letters.

Arithmetic Methods
We use the term “arithmetic methods” to describe all methods that do not relate in any way to trigonometric math. abs() You can calculate the absolute value of any number, integer or floating point, via this method. The absolute value of a number is its corresponding positive number; that is, if the number is positive, its absolute value is the number itself, whereas if it is negative, its absolute value is the same number with a + sign instead of a –. You can simulate this method easily:
function abs(num) { if (num < 0) return –num return num }

For example, the absolute value of –5 is 5. The absolute value of 5 is also 5. In JavaScript you can calculate the absolute value of a number via the Math.abs() method. This method returns the absolute value of its argument. You can use this method to compute the absolute value of only one argument. If you call the method with more than one, only the absolute value of the first is returned. If you want the corresponding negative value of a number, as opposed to the positive value, you can negate the returned value. For example:

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var neg1 = –Math.abs(–3.7) var neg2 = –Math.abs(3.7)

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You can use the Math.abs() for many purposes, not only in mathematical algorithms. ceil() The Math.ceil() method accepts a single numeric argument and returns the next integer greater than or equal to the argument (rounding up). Therefore, the returned value is never less than the argument. Here are a few examples:
Math.ceil(16) == 16 Math.ceil(16.01) == 17 Math.ceil(–15.01) == –15

Let’s say you need to fit num1 cars into parking lots, where each parking lot has space for num2 cars. You can use the Math.ceil() method along with a function to calculate the minimum number of parking lots:
function getNumLots(num1, num2) { return Math.ceil(num1 / num2) }

The reason we need to use this method in the above function is that you can only use a whole parking lot, not a fractional part; that is, we are trying to calculate the minimum number of parking lots needed, not the exact space. exp() This method returns Euler’s constant to the power of the specified argument (eargument). It is approximately equivalent to the following function:
function exp(num) { return Math.pow(Math.E, num) }

You refer to this function as Math.exp(). Here is an example:
document.write(Math.exp(4))

Its output is:
54.598150033144236

floor() The Math.floor() method returns the greatest integer less than or equal to the value passed to it. This is equivalent to integral division when dealing with non-negative numbers. It is also equivalent to rounding down to the nearest integer. Here are a few expressions, each evaluating to true:
Math.floor(16) == 16 Math.floor(16.01) == 16 Math.floor(–15.01) == –16

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log() This method returns the natural logarithm of the argument passed to it. For example, the natural log (base e) of e (Euler’s constant) is 1. You can confirm it via the following statement:
document.write(Math.log(Math.E))

And indeed, the output is 1. max(), min() Both of these methods accept two numeric arguments. max() returns the greater of two numbers, whereas min() returns the lesser of the two. Here is a function that prints the lesser of two numbers followed by the greater:
function printInOrder(num1, num2) { document.write(Math.min(num1, num2) + ", " + Math.max(num1, num2)) }

The following function call prints the string “–5, 1” to the document:
printInOrder(1, –5)

pow() Given two numeric arguments, this method returns the first one to the power of the second. Here are a few true expressions demonstrating the method:
Math.pow(10, 2) == 100 Math.pow(0.5, 0.5) == 0.7071067811865476 Math.pow(Math.SQRT2, 4) == 4.000000000000001

random() This method returns a random number between 0 and 1. It is obviously a floating-point number. The returned number’s precision is usually 16 digits after the decimal point. Here is a simple example:
for (var i = 0; i < 5; ++i) { document.write(Math.random() + "<BR>") }

The output of this loop depends on the output of random() and it is guaranteed that your results will not be the same as the following:
.924853870611902 .8248305636609181 .9539277224126104

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.9806934571332098 .7639888801207115

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This method is mostly used to create random integer numbers between x and y. Suppose x is 0 and y is a given number. You should multiply the value that random() returns by y and then round it off. For example, to generate a random number between 0 and 37 you can use the following expression:
Math.round(Math.random() * 37)

If you want an integer between 15 and 37 you can create a random integer between 0 and 22 and then add 15. Be very careful when attempting to create random numbers. round() The Math.round() method returns the nearest integer to the argument. If the argument’s decimal part is equal to 0.5, the number is rounded up. Here are a few true expressions to demonstrate the method:
Math.round(3.7) == 4 Math.round(4.5) == 5 Math.round(16.1) == 16 Math.round(0) == 0

If the argument is a negative number, the method returns zero, which happens to be the wrong answer. It would be better if an error was generated instead, because this wrong answer can go undetected.

Trigonometric Methods
Trigonometric methods are obviously those that deal with trigonometry. You should also know how to convert an angle from degrees to radians, and vice versa. Here is the basic conversion table:
Table 13-1. Degree-radian conversion table. Degrees 360 270 180 90 Radians 2 1.5 0.5

All angles in JavaScript are measured in radians, so the conversion table should help you visualize the size of an angle in radians.

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cos() The Math.cos() method accepts one argument, the angle of a triangle. It returns the cosine of that value, which must be specified in radians. The following statement prints –1:
document.write(Math.cos(Math.PI))

acos() The Math.acos() method also accepts one argument. It returns the arc cosine of the argument in radians; that is, it accepts the cosine of a certain value and returns that value—the opposite of the Math.cos() method. Therefore, the following statement prints the value of PI:
document.write(Math.acos(–1))

sin() The Math.sin() function returns the sine of its argument. Keep in mind that the argument must be in radian units. Here is a statement that prints 1:
document.write(Math.sin(0.5 * Math.PI))

asin() The Math.asin() method accepts one argument and returns its arc sine in radians. The following statement prints half the value of PI:
document.write(Math.asin(1))

From this script you can learn: n The sine of ¼ * PI is equal to its cosine which is also equal to the square root of ½. n The cosine and sine methods sometimes return inaccurate results. Take extra caution regarding the second concept—inaccuracy. The following statements:
var val = 0.25 * Math.PI if (Math.tan(val) == 1)

Inaccuracy is more obvious in this case, because the result differs even from browser to browser. The following is the output received from Microsoft Internet Explorer:
sine = 0.707106781186547 cosine = 0.707106781186548 tangent = 1

Notice that the sine and cosine values differ. Also notice that Internet Explorer appends a leading zero digit to all numbers between –1 and 1 (not inclusive). atan() As you could expect, the Math.atan() method returns the arc tangent of its argument. For example, the following returns one-fourth the value of PI:
document.write(Math.atan(1))

atan2() The Math.atan2() returns the angle (theta component) of the polar coordinate (r, theta) that corresponds to the specified Cartesian coordinate. You probably know that the normal x, y coordinates of a point are called Cartesian coordinates. Another measurement system is the polar system. You need to specify the point’s radius (distance from the pole) and angle (theta component).

The Number Object
The Number object is a built-in JavaScript object, and is very similar to the Math object in that it encapsulates several primitive numeric values. It is different from the Math object in that the Number object is dynamic and requires a creation step, while the Math object is static and does not requiring any instantiation. The primary use for the Number object is to access its constant properties, including the largest and smallest representable numbers, positive and negative infinity, and the Not-a-Number value. You can also use the Number object to create numeric objects that you can add properties to. It is unlikely that you will need to use the Number object, but it is given here for sake of completion. To create a Number object, use the following statement:
numberObjectName = new Number()

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where numberObjectName is either the name of a new object or a property of an existing one. To access Number’s properties, use the following format:
numberObjectName.propertyName

where numberObjectName is either the name of an existing Number object or a property of an existing object. propertyName is one of the properties listed in the next section.

Number Properties
The Number object has several properties. MAX_VALUE This is the maximum numeric value representable in JavaScript. The MAX_VALUE property has a value of approximately 1.79E+308. Values larger than MAX_VALUE are represented as “infinity.” Because MAX_VALUE is a constant, it is a read-only property of Number. The following code demonstrates the use of MAX_VALUE. The code multiplies two numeric values. If the result is less than or equal to MAX_VALUE, the func1 function is called; otherwise, the func2 function is called:
if (num1 * num2 <= Number.MAX_VALUE) func1() else func2()

MIN_VALUE The MIN_VALUE property is the number closest to zero, not the most negative number, that JavaScript can represent. MIN_VALUE has a value of approximately 2.22E–308. Values smaller than MIN_VALUE (“underflow values”) are converted to zero. Because MIN_VALUE is a constant, it is a read-only property of Number. The following code divides two numeric values. If the result is greater than or equal to MIN_VALUE, the func1 function is called; otherwise, the func2 function is called.
if (num1 / num2 >= Number.MIN_VALUE) func1() else func2()

NaN Unquoted literal constant NaN is a special value representing Not-a-Number. Since NaN always compares unequal to any number, including NaN, it is usually used to indicate an error condition for a function that should return a valid number. Notice, then, that you cannot check for Not-a-Number value by comparing to Number.NaN. Use the isNaN() function instead. Because NaN is a constant, it is a read-only property of Number. In the following code segment, dayOfMonth is assigned NaN if it is greater than 31, and a message is displayed indicating the valid range:
if (dayOfMonth < 1 || dayOfMonth > 31) {

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dayOfMonth = Number.NaN alert("Day of Month must be between 1 and 31.") }

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NEGATIVE_INFINITY This is a special numeric value representing negative infinity. This value is represented as “–Infinity.” This value resembles an infinity in its mathematical behavior. For example, anything multiplied by NEGATIVE_INFINITY is NEGATIVE_INFINITY, and anything divided by NEGATIVE_INFINITY is zero. Because NEGATIVE_INFINITY is a constant, it is a read-only property of Number. The following code extract checks a number for NEGATIVE_INFINITY and calls a different function if it is.
if (smallNumber == Number.NEGATIVE_INFINITY) func1() else func2()

POSITIVE_INFINITY This is a special numeric value representing infinity. This value is represented as “Infinity.” This value resembles an infinity in its mathematical behavior. For example, anything multiplied by POSITIVE_INFINITY is POSITIVE_INFINITY, and anything divided by POSITIVE_INFINITY is zero. Because POSITIVE_INFINITY is a constant, it is a read-only property of Number. The following code extract checks a number for POSITIVE_INFINITY and calls a different function if it is.
if (bigNumber == Number.POSITIVE_INFINITY) func1() else func2()

Number Methods
The Number object has no specific methods. You can still use the generic methods eval(), toString(), and valueOf(), which are applicable to every object.

Math-Related Functions
Although the functions presented in this section are also discussed later in the book, it is important that you attain a basic understanding, so do not skip the following explanation. These functions are used in examples later in the chapter. parseInt() This built-in function accepts a numeric string of an integer and returns its corresponding numeric value. The following example is worth a thousand words:
var numStr = "99" document.write("The initial " + typeof numStr + " is " + numStr)

The script’s output is:
The initial string is 99 The converted number is 99

The data types are not provided to the document.write() method as literals, but as values. Note that the parseInt() function accepts a string and returns an integer. If a noninteger numeric string is given to the function, it returns the nearest integer as a number (as opposed to a string). parseFloat() This function is exactly the same as the preceding one, except that it does not round off any number. It converts the numeric string to a number, without changing the numeric value at all. You should use this function with floating-point numeric strings, or when you are not sure what type of number is being used. eval() The eval() function is both powerful and useful. For now, you just need to know that upon acceptance of a string representing a numeric value or mathematical expression, it returns the value to which the expression evaluates. The following script segment prints “13”.
var str = "6 + 7" document.write(eval(str))

The function returns a number, not a string.

Math Examples
Creating a Curve
Although JavaScript does not correspond very well with custom-created graphics, it is possible to create simple graphs, plots, and curves. In this section we shall plot a sine curve using JavaScript. First take a look at the script’s output:

The script is intentionally divided into many short functions for better understanding by the reader. We shall analyze each function to see how it works. drawBlank(num)
function drawBlank(num) { // draw num blank dots for (var i = 0; i < num; ++i) { document.write("<IMG SRC='blank.gif' HEIGHT=6 WIDTH=6>") } }

This function uses a for loop to print the transparent image the number of times indicated by num. Note that the transparent (blank) image’s height and width are both equal to 6. drawDot()
function drawDot() { document.write("<IMG SRC='dot.gif' HEIGHT=6 WIDTH=6>") }

drawBlank(30) // print current degree printDeg(deg) } // move to next line document.write("<BR>")

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return Math.PI * deg / 180 }

This function accepts a number (actually the size of an angle in degrees) and returns it in radians. The conversion is based on a simple linear equation. getSpot(deg)
function getSpot(deg) { // convert from degrees to radians var rad = getRadian(deg) // assign sine to variable var sine = Math.sin(rad) // return spot in graph return Math.round(sine * 30) }

This function accepts the size of an angle in degrees and returns its sine value multiplied by 30. At first, it assigns the radian measurement of the angle to the local variable rad. It then assigns the sine of that angle to the local variable sine. The function then multiplies the sine by 30 and returns the rounded-off value of that number. The reason for the multiplication by 30 is that the sine of any angle is a value between –1 and 1. The curve we are plotting generally consists of 30 images, transparent or not, at each side of the center axis. If you increase this number, the horizontal width of the curve becomes larger. As you can see, 30 images looks good. By multiplying a value between –1 and 1 by 30, we receive a value between –30 and 30. By rounding it off using the Math.round() method, the function returns an integer between –30 and 30. Note that the plot acts like a 2D array, because it consists of rows of images. get3DigitNum(num)
function get3DigitNum(num) { // convert num to string num += "" // assign number of digits in num to variable var length = num.length // add preceding zero digits to reach three digits for (var i = 0; i < 3 – length; i++) { num = "0" + num } // return three-digit number return num // do not parse number! }

This function accepts any single-digit, double-digit, or triple-digit number, and returns a triple-digit number, appending preceding 0 digits when needed. At first, it converts the number to a string, required to use the length property. A string type is also required because numbers with leading 0 digits are considered octal. The length, or

the number of digits in the original number, is assigned to the local variable length. The following statement is a loop whose command block is executed once for each digit missing to complete a triple-digit number. For example, if the number consists of two digits, the loop’s command block is executed once (3 – 2 = 1, 1 – 0 = 1).

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document.write("<BR>") }

The rounded-off sine value multiplied by 30 is assigned to the variable spot. The function is basically divided into three sections, only one of which is executed. The first section deals with a situation in which the sine value is negative, the second deals with positive sine values, and the third deals with the remaining situations, that is, when the sine is zero. When the sine (sine multiplied by 30) is negative, 30 – spot transparent images are printed. For example, if the value of spot is 20, 10 transparent images are printed. The dot image is printed after these transparent images. The function then prints –spot – 1 transparent images, up to the center axis. So far, 30 + spot transparent images have been printed, as well as one more image representing the dot on the curve. In total, there are 31 + spot images. Remember that each side of the axis consists of 30 images, so 30 – (31 + spot) images still need to be printed; that is –spot – 1. The current degree is then printed via a call to the function printDeg(). When the sine of the current angle is positive, 30 transparent images are printed to fill up the left side of the center axis. The current degree is then printed to continue the vertical span of the axis. Then spot – 1 images are printed, up to the place where the dot on the curve needs to be placed. The dot image is then printed. When the sine of the current angle is zero, a row of 30 transparent images is printed, followed by the current angle in degrees. The last statement of the function appends a line break to the document, opening a new row of images. drawCurve(lastDeg, jump)
function drawCurve(lastDeg, jump) { // loop through plot vertically for (var deg = 0; deg <= lastDeg; deg += jump) { drawLine(deg) } }

This function accepts two arguments. The first parameter, lastDeg, accepts the last value included in the curve, that is, the last angle whose sine appears. The second argument specifies the difference between each two angles. Global Statements The script includes only one global statement, a function call. It calls the drawCurve() function with the desired arguments.

Figure 13-2. A confirm box presented by the wizard displays a welcome message.

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Figure 13-3. A confirm box that displays a list of supported functions.

When you load the page, the “wizard” pops up. The wizard displays the basic features and usage instructions via a few confirm boxes. If the user presses Cancel on any of the confirm boxes, the wizard is terminated, and the actual plotting application begins. The first output of the plotter is a confirm box requesting the function, which may be constructed out of JavaScript’s supported mathematical functions, constants, and operators. If the user selects Cancel, an alert box displays an error message. Otherwise, the execution continues, and JavaScript asks the user to enter the minimum value on the X axis. Cancellation of this box results in the termination of the script. If the user selects OK, the execution continues, and another prompt box is presented. Once again, if the user presses Cancel, the script is terminated. Otherwise, the user is asked to enter the difference between two consecutive ticks on the X axis (e.g., if the user wants to see –4, –2, 0, 2, 4, 6, then the difference is 2). Now that we know exactly what the script does, let’s see how it does that by analyzing each function. root(a, b)
function root(a, b) { // return b to the root of a return Math.pow(b, 1 / a) }

This function is responsible for the wizard. At first, an array is created as an instance of the Array() object. Four different messages are assigned to the array as elements. The first is stored in ar[0], the second in ar[1], and so on. Note that the assigned strings are very long, so the assignment operator concatenates them by assigning them, one after the other, to the element of the array. The function uses a for loop to print the messages to the confirm boxes. If the user presses Cancel, the confirm box evaluates to false, and the break statement is executed, causing the loop to terminate immediately. drawBlank(num)
function drawBlank(num) { // draw num blank dots for (var i = 0; i < num; ++i) { document.write("<IMG SRC='blank.gif' HEIGHT=4 WIDTH=4>") } }

This function basically prints the dot image, but it prints it as a hypertext link. The URL specified as the link is a JavaScript statement. When the user clicks the image, an alert box is displayed, presenting the coordinates of that dot. The coordinates are given to this function as arguments. For example, if the arguments are –5 and 10, the following string is printed to the document:
<A HREF="javascript:alert('X = –5, Y = 10')"><IMG SRC='dot.gif' HEIGHT=4 WIDTH=4 BORDER=0></A>

This function matches certain strings in the argument and replaces them with other strings. Here is an example:
str = str.split("max").join("Math.max")

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The string is split into an array where the substring max is the delimiter, and the elements are joined back to a string with a delimiter, Math.max. The final result is that, wherever the string max appears in the main string (str), it is replaced by the string Math.max. This is important because the string must be constructed as a valid JavaScript expression (the eval() function evaluates a string consisting of a valid JavaScript expression). The same process is done with all supported constants and mathematical functions. The function factorial() remains untouched, because its reference in the script is also factorial(). An important concept to notice is that the order in which substrings are replaced is vital. For example, you cannot replace the sqrt substring with Math.sqrt before you replace sqrt2 with Math.SQRT2, because the result would be that the substring sqrt2 is replaced by Math.sqrt2, which is not Math.SQRT2. The function must first replace subscripts that contain other subscripts that need to be replaced. After all desired substrings are replaced, and provided the user entered a function in a valid format, the function returns the formatted string, which is now a valid JavaScript expression. splitFunc(func)
function splitFunc(func) { // exclude "y =" from func var expr = func.substring(func.indexOf("=") + 1, func.length) // replace regular trigonometric functions with JavaScript convention expr = replaceSpecialSequence(expr) // split argument to *global* array, excluding "x" chars funcArray = expr.split("x") }

This function accepts one parameter—the function. As you know, the general format of the function is y=f(x). At first, the function assigns everything after the equal sign to the local variable expr. It then sends the value of expr to the function replaceSpecialSequence() and accepts a string in return in which the special substrings were replaced (see explanation about that function). A global array, funcArray, is created by assigning it a value without using the keyword var. The string stored in the variable expr is split into the funcArray array, using the character x as a delimiter. Let’s say the function’s argument is y = x * sin(x). The variable expr is assigned x * sin(x). This value is sent to the replaceSpecialSequence() function, and the string x * Math.sin(x) is assigned to expr. This string is split by x to the global array, funcArray. Its elements are now:
funcArray[0] == " " funcArray[1] == " * Math.sin(" funcArray[2] == ")"

This function is responsible for prompting the user for the function he or she wants to plot. At first, the function, as entered by the user in a prompt box, is assigned to the local variable input. If the user entered a value and selected OK, the following command block is executed. This command block consists of a call to the function splitFunc(). It also includes a statement that prints the function to the document as a bold level-1 header. After the Boolean value true is returned to indicate that the user entered a value and pressed OK, the function is terminated. If the user selected Cancel, the command block is not executed, so the execution continues immediately after the block. An error message is displayed in an alert box, and the value false is returned. yVal(xVal)
function yVal(xVal) { // build expression with xVal instead of x in original equation var expr = funcArray.join(xVal) // return evaluated expression return eval(expr) }

This function accepts a value representing the x coordinate of a dot on the curve and returns its corresponding y coordinate. The first statement declares a local variable, expr. It joins the elements of the global array funcArray back to a string, with the value of xVal between each two. Refer back to the function splitFunc() and read the example given (y = x * sin(x)). The values of the elements of the array are the following:
funcArray[0] == " " funcArray[1] == " * Math.sin(" funcArray[2] == ")"

The function yVal() joins these elements using the value of xVal as a glue between each two. Let’s say the value of xVal is 5. The constructed string assigned to expr in this case is:

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" 5 * Math.sin(5)"

The function returns the evaluated value of this string. For the preceding string, the value returned is:
" 5 * Math.sin(5)" == –4.794621373315692

makeArray() is a constructor function. It creates an object consisting of properties only. The names of the properties are the x values of the points that are to be drawn on the curve. Their value is the corresponding y coordinates of the points. The function accepts three arguments, which are all entered by the user via prompt boxes outside this function. The function creates the properties of the calling instance via a loop. The loop counter starts at min and is incremented by dif after iteration. The loop continues as long as its counter is less than or equal to max. The corresponding y coordinate of each x coordinate is retrieved by the function yVal(). printUnit(num)
function printUnit(num) { // print number in specified font and size, inside a table cell document.write("<FONT SIZE=1 COLOR='purple'>" + num + "</FONT>") }

This function does not accept any argument. It is responsible for printing the curve. To understand this function, you must first understand how each row of the bitmap is printed. The entire row is placed inside an HTML table. The x value, or unit, is printed in the first cell of the table. This cell’s width is fixed in order to preserve uniformity across rows. The second, and last, cell of the table is located on the same row, and contains all the images, both the transparent ones as well as the dot. The dot image is obviously the last image on the second cell of every row. Note that the left cell (the first in each table) is wide enough to hold almost any number. The entire function is built of a single loop, which iterates through the points that are to be placed on the curve. See the explanation on the function makeArray() for information on the loop’s algorithm. At first, the function prints the opening <TABLE>, <TR>, and <TD> tags. The border of the table is set to zero, so it is not seen. The width of the cell is set to 90 pixels. The value in the first cell is printed by the printUnit() function. The current cell of the table is then closed, and a new one is opened. The y value of the current point, which is stored as a property in the global object ar, is assigned to the local variable y for the sake of convenience. The minimum and maximum y coordinates of the whole curve are already stored in the global variables minY and maxY, respectively. A simple linear equation is used to convert the current y coordinate to a value between 1 and 100, where the minimum y coordinate in the entire curve is 1 and the maximum is 100. This value is assigned to the local variable yspot. The following statement calls the function drawBlank() to print ySpot – 1 transparent images, which is equal to the number of images up to ySpot, the spot where the dot image is placed. The function drawDot() is called with the current coordinates to print the current dot, or bullet, on the imaginary curve. The current cell and row of the table are closed, and so is the table. When closing the table, a line break is automatically appended. main() The name of this function is taken from the C++ language, where the function main() is executed automatically. In JavaScript you have to call main() to draw the graph. At first, the function calls the startWizard() function to execute the wizard. The function getInput() is then executed. If it evaluates to false, meaning the user pressed Cancel when asked to enter a function, the function is terminated. Otherwise, execution continues. The function asks the user to enter additional information, including the minimum and maximum x coordinates and the difference between two consecutive ticks on the X axis. If the user selects Cancel on any of these requests, the function is terminated immediately, without executing any additional statements. After the user enters the values (and presses OK), the data is sent to the function replaceSpecialSequence from which a string is returned and converted to a number with the eval() function.

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The following statement creates a global array-like object and assigns values by its constructor function, makeArray(). The function now needs to find the minimum and the maximum y coordinates of the curve. First, it assigns each variable, maxY and minY, an existing y coordinate. A for…in loop then iterates through the properties of the ar object, and if it finds a value less than the current minY or greater than the current maxY, it updates that value. The last statement of the function draws the curve itself. Global Statements The only global statement is a call to the main() function.

Summary
In this chapter you learned about math usage in JavaScript. We have mostly discussed the Math object and its properties and methods. Among the methods discussed are the arithmetic and trigonometric ones. Arithmetic methods are found in almost every JavaScript script, whereas trigonometric methods are rather rare. You also grasped the basics of some built-in functions that are closely related to JavaScript. These functions are discussed later in depth, so do not worry if you do not understand them very well. To spice things up, we have also shown some interesting examples related to math, one involving curve plotting. If you understood all the scripts, or at least the curve plotting one, you are surely on the right track. However, don’t worry if they seem very difficult at this stage, because knowledge comes with practice. In the following chapters we shall continue to learn about JavaScript’s unique object model, including built-in objects and browser objects.

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Handling Strings
String Indexing
Essentially strings are simply an array of individual characters. This is true in all programming languages. Some languages, such as C and JavaScript, have you deal directly with the array of characters, whereas other languages, such as Visual Basic, hide that level of detail from the programmer. Each character has its position, or index. The indexing of strings, like that of arrays, is zero-based. The index of the first character of a string is 0, the index of the second one is 1, that of the third one is 2, and so on.

Characters
JavaScript, unlike some languages, does not include an explicit character (char) data type. A character is simply a string constructed of only one character. Characters of a string must be visual. That is, each symbol that appears in a string is a character. However, not all characters that appear in the ASCII table can be characters of a string, because some are not visual. An escape sequence is a sequence of characters that represents a single special character, which is usually difficult to enter via the keyboard. For example, take a look at the following string:
"car\'s wheel"

It appears as if the string contains 12 characters (c, a, r, \, ', s, , w, h, e, e, l). However, when you print the string you can see only 11 characters (c, a, r, ', s, , w, h, e, e, l). Therefore, the string consists of 11 characters. The fourth character is an apostrophe, not a backslash, because the sequence “\'” is shorthand for an apostrophe.

Creating Strings
String is a very special built-in object, because of the way you can create it. All strings belong to the built-in String object, so you can create strings as instances of the String object:
var str = new String("Hello!")

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The general syntax is:
stringObjectName = new String(string)

stringObjectName is the name of the object you are creating. string is any string, including literal strings in quotations as in the example. Strings created via this constructor function are considered objects. If you test their data type using the typeof operator you will find that they are objects, not strings. However, all the string properties and methods work on such objects. Another way to create a string is by simply quoting a sequence of characters. If you want to use it as an object, you can follow the identifier by the desired property or method.

The index of a string’s last character is equal to its length (number of characters) minus one. The string’s length is the index of the non-existing character following the string’s last character.

HTML Formatting Methods
You have seen a great amount of HTML formatting throughout the book. You can print HTML tags by combining strings to receive the desired tag and attribute values. Here is an example:
var openBold = "<B>" var closeBold = "</B>" var message = "Something" document.write("<B>Something</B>")

The result of this script is that the string “Something” is printed in bold. JavaScript provides methods to simplify this process. For example, you can create the same output using the following statement:

You can “chain” methods together in order to apply more than one formatting conversion. For example, if you want an italic bold uppercase string, you can use the following expression: toUpperCase().bold().italics(). The evaluation here is done from left to right. The following list outlines the stages of the evaluation, where the calling string is the literal "text":
"text".toUpperCase().bold().italics() "TEXT".bold().italics() "<B>TEXT</B>".italics() "<I><B>TEXT</B></I>"

The value processed by a specific method is the accumulative value returned by the expression to the left of the method; that is, the expression that is implemented as the calling object, or string. Therefore, you must make sure that the expression to the left of a method returns a value which is valid for that method. HTML text formatting tags usually consist of two tags that enclose the text. The nested structure is very clear in HTML, because a set of tags can enclose another set of tags. In the previous example, the <I></I> set encloses the <B></B> set. When creating HTML via JavaScript’s String methods, keep in mind that the far-left specified method appears as the inner set of tags when formatted to HTML, whereas the far-right method is responsible for the outer set of tags.

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General String Methods
As you already know, the String object packs many methods. Those that convert strings to constant HTML formats (listed in Table 14-1) are only some of the methods JavaScript offers. In this section, we will take a look at the rest of the String methods.

charAt()
This method returns the character whose index is equal to the argument of the method. The characters of a string are indexed from 0 to length – 1. The general syntax is:
anyString.charAt(index)

This script segment prints the character “e”, because it is the second character in the string. You can also call this method with a literal as in the following example:
document.write("Kennedy".charAt(1))

You can print the characters of a string via a simple loop:
var str = "I am a string!" for (var i = 0; i < str.length; ++i) { document.write(str.charAt(i)) }

At first, a string literal is assigned to the variable str. The loop then iterates length times. It starts at 0, and ends at str.length – 1 (notice the less than operator, not less than or equal to). The i indexed character is printed in the ith iteration of the loop. Since the command block is executed once for each integer value of i, each character of the string is printed once and only once. The output of the preceding script segment is actually the string itself, printed one character at a time (no, you cannot notice the difference!).

indexOf()
This method returns the index of the first occurrence of the specified substring in the calling String object, starting the search at the beginning of the string. An example will surely clear things up:
var str = "ababa" document.write(str.indexOf("ba"))

This script’s output is the number 1. The first occurrence of the substring “ba” in the calling String object is at the second character whose index is 1. The search for the specified substring starts at index 0, the beginning of the string. However, you can

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also instruct JavaScript to start the search at a different index, somewhere in the middle of the string. The following script segment prints the number 3:
var str = "ababa" document.write(str.indexOf("ba", 2))

The general syntax of this method is:
stringName.indexOf(searchValue, [fromIndex])

Note that fromIndex must be an integer between 0 and the string’s length minus 1. searchValue does not have to be a string. The following script prints 8:
var str = "August 27, 1985" document.write(str.indexOf(7))

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If the index to start looking for the substring is not specified, the default value which is 0, is used. If the specified substring is not found in the entire string, the method returns one less than the base, –1. This method is equivalent to the index function in Perl.

lastIndexOf()
This method is identical to the indexOf method, except that it returns the index of the last occurrence of the specified value, rather than the first occurrence. Its syntax is, therefore, the same:
stringName.lastIndexOf(searchValue, [fromIndex])

The following script prints the number 3:
var str = "a/b/c" document.write(str.lastIndexOf("/"))

See the indexOf method for more details.

substring()
Strings are constructed of characters. The substring() method returns a set of characters within its calling String object. Its general syntax is:
stringName.substring(indexA, indexB)

stringName is any string. indexA and indexB are both integers between 0 and stringName.length – 1. indexA is the index of the first character in the substring, whereas indexB is the index of the last character in the substring plus 1. The following script assigns the string “bc” to the variable seg:
var str = "abcd" var seg = str.substring(1, 3)

Notice that the length of the substring is indexA – indexB. The substring whose arguments are 0 and stringName.length is equal to the string itself (stringName).

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This method is similar to its equivalent in Perl, the function substr. Nonetheless, it is important to point out the differences. First of all, since Perl does not support objects, the plain substr() function accepts the string itself as the first argument and indexA as the second one. However, the third argument is not indexB, but the length of the substring. Another difference is that when you call the function with a negative value as the offset (indexA), the substring starts that far from the end of the string. In JavaScript, though, a negative index is equivalent to a zero index, the first character of the string. JavaScript prototypes enable us to reproduce the substr function in Perl as a JavaScript method using the following script segment:
function substr(offset, length) { if (offset < 0) offset = this.length + offset return this.substring(offset, offset + length) } String.prototype.substr = substr

You can use this method with any string in the following way:
var str = "abcd" document.write(str.substr(–3, 2))

Like all methods, you can pass it a variable, a property of an existing object, or a plain string literal. The escape() function is not a method associated with any object, but is a part of the language itself. The value returned by the escape function is the string argument, where all nonalphanumeric characters are replaced by a string in the form of “%xx”, xx being the ASCII encoding of a character in the argument. The unescape() function is responsible for the opposite conversion. That is, it converts the string from nonalphanumeric ASCII encoding to ISO Latin-1 characters. Its syntax is similar:
unescape(string)

The script’s output is self-explanatory:
After escape: My%20phone%20%23%20is%20123-456-7890 After unescape: My phone # is 123-456-7890

Number-to-String Conversion
Occasionally, you need to convert a number to a string. For example, if you want to compute the number of digits in a number, you can convert it to a string and use the length property, which applies only to strings. In this section we take a look at a few ways to convert a number into a string.

Empty String Concatenation
The most obvious way to convert a number to a string is by concatenating an empty string to the number. Here is an example of such a conversion:
var num = 987 num += ""

You can also make sure that the value of the variable is a string using the typeof operator in the following way:
var num = 987 document.write("num is a " + typeof num + "<BR>") num += "" document.write("num is a " + typeof num)

The expected output of this script segment is:
num is a number num is a string

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You can also convert the number to a string and assign the numeric string to another variable, or even better, do both operations in one statement:
var num = 987 var numericString = num + ""

This script results in two different variables; the first holds a pure numeric value and the second, numericString, holds a string type. The side of the variable to which the empty string is concatenated has no importance:
var num = 987 var numericString = "" + num

If you concatenate several different literals, where some are numbers and others are strings, the expression evaluates to a string. Here is an example:
var str = 99 + " bottles of beer on the wall"

However, scripts become tricky when you concatenate more than two values or literals, especially when the first few are numbers. Here is a tricky expression:
var str = 50 + 49 + " bottles of beer on the wall"

JavaScript evaluates from left to right. The accumulated value is converted to a string only when a string value or literal is encountered in the expression. In the preceding example, JavaScript adds 50 to 49 in the regular mathematical way, so 50 + 49 evaluates to 99, which is then concatenated to the string that follows. So the value of str in this case is “99 bottles of beer on the wall”. The following statement demonstrates a slightly different situation:
var str = "bottles of beer on the wall -- " + 50 + 49

Like always, evaluation is done from left to right. The string, "bottles of beer on the wall -- ", is concatenated with 50 and evaluates to "bottles of beer on the wall -- 50". This value in turn is concatenated with the number 49, and evaluates to "bottles of beer on the wall -- 5049", which is certainly not the value we want. A simple workaround is to enclose the numeric operation in parentheses in the following form:
var str = "bottles of beer on the wall -- " + (50 + 49)

The parentheses instruct JavaScript to evaluate the enclosed expression first, so the value of str in this case is "bottles of beer on the wall -- 99".

String Instance Construction
Another way to convert a number to a string is by providing the number to the String() constructor function, which returns a regular String object. Here is a simple example to demonstrate this:
var num = 987 num = new String(num)

The data type of the variable num in this case is not a string, but an object. As mentioned earlier, strings created via the String() constructor are regular objects.

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However, you can still use any property or method associated with strings on such objects. A more obvious way to convert a number to a string via the constructor function is to assign the new string, or object, to a new variable in the following form:
var num = 987 var numericString = new String(num)

The toString() Method
The toString() method belongs to all objects. Its general syntax is:
objectName.toString([radix])

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objectName is the object to convert to a string, whereas radix is the base to use for representing numeric values when the calling object is a number. The following example prints the string equivalents of the numbers 0 through 9 in decimal and binary:
for (x = 0; x < 10; x++) { document.write("Decimal: ", x.toString(10), " Binary: ", x.toString(2), "<BR>") }

All objects, numbers included, have a toString() method. If an object has no string value, the method returns “[object type]”, where type is the object type (e.g., Date, Array, Object (user-defined), Image). When used with an array, toString() joins the array elements and returns one string in which elements are separated by commas. This operation is exactly like the join() method which concatenates the elements with a specified delimiter, possibly a comma. For functions, toString() decompiles the function back into a canonical source string. Take a look at the following script segment:
function foo() { var a = 5 alert(a) document.write("wow") } document.write(foo.toString())

This script prints 5, just as if both variables were assigned a plain numeric value rather than a numeric string (we use the term numeric string to characterize a string that encloses a number, such as “911”). An operation consisting of numeric string operands returns a plain numeric value, not a string. Therefore, you can theoretically convert a numeric string to a number by performing an arithmetical operation on it. If you want, you can even use a function to execute the conversion in the following form:
function convert(val) { return val – 0 } var num = "911" num = convert(num)

Note that you cannot use the plus (+) operator because it is also a string concatenation operator in JavaScript. If you are not sure whether or not a value is a numeric string or a number, always convert it. It’s better to stay on the safe side than to spend hours searching for such errors. Conversion via mathematical operations is somewhat annoying, because it looks like a workaround. Therefore, JavaScript provides us with a few conversion functions, each with its own attributes.

parseInt() and parseFloat()
These two functions were briefly discussed in Chapter 13, JavaScript Math. They are built-in functions, so they do not belong to any object. They convert their argument from a numeric string to a number. If the argument is a string but not a numeric one, the function returns zero. The parseFloat() function is more general, because it works with floating-point numbers as well as integers. The parseInt() function works with integers only, and returns a rounded-off value when called with a floatingpoint numeric string. Both functions return the value they are given if it is a plain number, not a numeric string. Therefore, if you are not sure whether a value is a number or a numeric string, simply send it to the function. If a certain value in the script has a chance to be a floating-point number, use parseFloat. It will also work if the value is an integer.

These functions are very useful when accepting input from the user via forms or prompts, because they always return a string, even if it represents a number. Note that both functions return zero when the argument is a Boolean value. Thus, if you want to check if the user canceled a prompt box by pressing the Cancel button, you must evaluate the condition before parsing the value. Here is an example:
var num = prompt("Enter a number between 0 and 9:") if (num = false) alert("You must enter a value to continue.") else num = parseInt(num)

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A common but mistaken practice is to parse the value immediately. The result is that you cannot check if the user canceled the box, because he or she might have entered the number 0, which is parsed to the same value as a Boolean value.

Determining if a Value is a Number or Not
The isNaN() function evaluates an argument to determine if it is not a number, or NaN. The functions parseFloat() and parseInt() return NaN when they evaluate a value that is not a number or a numeric string. NaN is not a number in any string. If NaN is passed on to arithmetic operations, the result is also NaN. The isNaN() returns a Boolean value, according to the argument. Bear in mind that Internet Explorer 3.0 does not support this feature—parseFloat() and parseInt() both return 0 if their argument is neither a string nor a numeric string. However, this is supported in the current version. NaN is not a string, nor is it a data type of its own. It is primarily a number! You can prove that to yourself via the following statement:
alert(typeof parseInt("something"))

The following construct demonstrates how to implement the isNaN function (with the parseFloat() function for the sake of the example):
var floatValue = parseFloat(valueToBeConvertedToFloat) if isNaN(floatValue) { functionToBeCalledIfFloat() } else {

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functionToBeCalledIfNotFloat() }

The isNaN() function is not as important as parseInt and parseFloat, but we have discussed it here for completeness.

This script segment pops up two alerts. The first one displays the number 7, because the expression “5 + 2” evaluates to 7. The second call to the eval() function does not cause it to return a value, but to execute the statement encapsulated in a string. You can also use the eval() function to convert strings representing numbers to regular numbers. The eval() function accepts any valid JavaScript piece of code in the form of a string. You can store an entire script as a string and then hand it over to this function. The classic example for the function is to let the user enter a mathematical expression in a form field or a prompt box, and to display the result. Here is a simple example:
var inp = prompt("Enter mathematical expression", "") alert(inp + " = " + eval(inp))

String Handling Example
In this section we focus on a script that takes advantage of the various built-in functions and other elements of the String object in JavaScript.

String Enciphering
The following script prompts the user for a short string. It then asks for a numeric key. The key has 63 possible values—all integers from 1 to 63. The ciphering technique used in this script is known as XORing, because it is primarily based on the bitwise XOR (exclusive OR) operator. The numeric value of each character of the input string, or password, is mixed with the numeric key value. A reverse process can be used to convert the enciphered string back to the original one. Since the conversion simply swaps the same two characters according to the key, the same JavaScript script is used as the decoder and the encoder. Enough theory—let’s get to the point! Here is the script:

Warning: This is an incredibly primitive encrypting scheme and can easily be broken. It is presented here only as an example of how to use JavaScript and is not meant to be used to encrypt sensitive data.

encipher() At first, the encipher() function prompts the user for the string he or she wants to encode. It is stored in the variable str. If the value of the variable is a Boolean false, the function and the script itself are terminated. The motivation behind terminating the function is that a Boolean false value can only be the result of the user pressing Cancel. A for loop is then used to check that all characters of the string are also located in the string held by list. The loop iterates through every character of the input string. Take a look at the condition used to test if the character is supported:
list.indexOf(str.charAt(i)) == –1

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str.charAt(i) is the character for which the loop’s block is currently being executed. The variable i starts at 0, the index of the string’s first character, and is incremented each time until it is equal to the index of the string’s last character. Suppose the current character is “t.” The condition looks like this then:
list.indexOf("t") == –1

If the character “t” is not found in the string list, the method indexOf() whose argument is “t” returns –1—exactly the number against which the returned value is tested. If a character is not valid (not found in list), a message is displayed and the function is terminated, indirectly causing the script’s execution to end. The function then asks the user to enter the key number, which must be an integer from 1 to 63. Because this is just an example, the input value is not tested. If the user clicks Cancel, the function is terminated. Otherwise, the function continues, and the key number is converted from a numeric string to a number via the parseInt() function. The encoded string, which is returned by the function encode(), is displayed. encode(str, key) At first, an empty string is assigned to the variable code. A loop is used to replace every character of the input string by another character. The index of the current character (the one whose index in str is equal to the loops counter, i) in the list string is assigned to the variable ind. The bitwise OR operator is given the key number and the value of ind as operands. The character whose index in list is equal to the value returned by the bitwise OR operation is the one used to replace the current character in the new string, so it is assigned to the variable that holds the encoded string. The new string is returned by the function.

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Summary
Strings are a very useful data type in JavaScript. JavaScript tends to organize its elements in the form of objects, so all string-related functions and data are grouped together to form the String object. In this chapter we discussed the methods of the String object, as well as its single property, length. Because strings are direct or indirect (based on the way you create them) instances of this object, you can create prototypes to extend the object’s capabilities. The JavaScript example provided at the end of the chapter gives a clear view of the String object’s strength. We did not provide many examples in this chapter because string manipulation and handling can be found in almost every example later on in this book.

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Chapter 15

Browser Objects
Browser objects are special objects that correspond to elements of a page. These objects exist on all pages, even if the page is “empty.” Each browser object corresponds to one element of the page. For example, the location object consists of all the properties related to the current URL of the document. Browser objects are also known as Navigator objects, but that term is specific to Netscape’s browser. Some even refer to them as built-in objects, but this term is incorrect because built-in objects are another explicit type of objects (Date, Array, Math, String). Although browser objects exist on all pages, they consist of nested objects that are largely content dependent. Browser objects are the only type of objects in JavaScript that feature event handlers as a part of their structure. Although these objects are only referred to using JavaScript, they are not created via JavaScript but by HTML.

The Object Hierarchy
You may recall that objects often consist of properties that are also objects. The term nested objects is usually used to characterize such child objects. The nested structure of an object is also known as its hierarchy. The highest-level object is the one that does not have any parent object. Browser objects are classic examples for hierarchy usage because they are deeply nested. The browser object structure is fairly complex, so it is difficult to remember each and every property at the bottom of the tree. Nonetheless, it is important to study the upper section of the hierarchy. Figure 15-1 shows the upper part of the browser object hierarchy. The window object is the topmost object in the entire structure. Everything you script in JavaScript refers to elements of the browser’s window, whether or not it appears in the page itself. Normally, it is not necessary to specify the window object when you refer to its properties. For example, the document object is a property of window, but you say document.write rather than window.document.write. The implementation of the window object as the default object throughout the script is influenced by the fact that it is solely the topmost level of the hierarchy. Once you establish a mental model of the browser hierarchy and script these objects a few times, the structure will become second nature to you.

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Figure 15-1. The browser object hierarchy.

Creating Browser Objects
As you know, you do not create browser objects via JavaScript. JavaScript is only the high-level scripting language used to refer to these objects. Browser objects existed even when the most popular browser was Mosaic or Navigator 1.0, but they were not originally implemented to facilitate interaction with the user. Since there was no way to refer to these objects, they were not stored in memory in these earlier versions. The topmost browser objects are established when the HTML document loads in the Java- or JavaScript-enhanced browser. JavaScript lets us handle browser objects via their properties, methods, and event handlers. Bear in mind that browser objects exist in memory as they are created by the browser. You can refer to a browser object via JavaScript only if the specified object has been laid out already. Otherwise, JavaScript will generate an error reporting that the object does not have any properties. At that moment, it is true that the object does not have properties, because the elements of the object haven’t been laid out yet. Keep in mind that browser objects are visible objects. For example, a form’s field is a browser object, and it is visible. Because browser objects are actually made of HTML, they include event handlers that enable a response to actions by the user, who is interacting with those elements of the page.

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The Topmost Objects
The window object is standing alone at the top of the browser hierarchy. The properties of this object are, therefore, very rudimentary. In this section we will outline the most important properties of the window object.

window
This object is the top-level object of the hierarchy. It contains properties that apply to the entire window. For example, the status bar of the browser is a property of this object. It also includes a property that is actually the object itself. Sound strange? Not really. You will find out more about this reference when we discuss windows and frames. When the document features frames, there can be several window objects in a

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single HTML document. Frames actually divide the page into child windows, so each frame has its own browser object hierarchy. You must be careful with such child windows because they can cause a collision due to the fact that several window objects have a shared property. For example, there is only one status bar in the browser, no matter which page you load (unless you open a window without a status bar). However, there can be many window objects in action on that page, each one of them optionally referring to the same status bar. You will learn more about the window object later in the book.

document
By far, the most useful property of the window object is the document object. It contains properties for the current page loaded in the window. The properties of this object are content dependent because each and every page has its own outline and elements. Almost everything in the page is a property of the document object, including links, images, forms and their elements, anchors, and more. Because each frame is a window (window object), it contains a document object as well. Even the background and the title are properties of this object. As a matter of fact, the document object is so complex that it is divided into several chapters in this book, each dealing with different properties of this object.

history
The history object is also a property of the window object. It contains properties of the URLs the user has previously visited. This information is stored in a history list, and is accessible through the browser’s menu. If the document consists of child documents in frames, each frame has its own history list, or history object. This object also contains methods enabling you to send the user’s browser to a URL found in the history list.

location
The location object contains properties of the current URL. Although you may think of URLs as simple standard structures, they are far from being that. There are many types of protocols and various sections in every URL. There are also optional sections in the URL such as anchor names and queries. The following image summarizes all the browser objects:

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In Figure 15-2 you can see the various top-level browser objects, except for those that deal with frames. Study the image carefully so you know the role of every object.

Summary
In this short chapter we have introduced the browser object as a very important model. Mastering browser objects is the key to success in JavaScript, because JavaScript is primarily designed to enable interaction with the user via these objects. Although we did not discuss any specific browser object, you should have a clear picture of the hierarchical structure. In the following chapters we discuss various concepts related to browser objects, including their methods, properties, and event handlers. Browser objects are not dealt with specifically in these chapters, but rather they are presented as the key to many JavaScript topics.

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Figure 15-2. A standard web page.

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Utilizing the Status Bar
The Status Bar
The status bar is found at the bottom of the browser’s window. It exists in all browsers, including Navigator and Internet Explorer. It normally displays the current status of the document being loaded. The status bar is present in every window, unless the window is opened by a script that explicitly disables the status bar. Figure 16-1 shows the status bar, the gray bar at the bottom that shows the message “Fascinating!” In terms of JavaScript, the status bar is a direct property of the window object. In multiple-frame documents each frame has a property representing the status bar, but you should only reference one of them to avoid an unwanted collision. The status bar in JavaScript is generally represented by the status property window.status. You can also refer to the status bar of the current window via self.status, because self is actually a property of the window object that is the window object itself.

þ

Note: You can also refer to the status bar as status instead of window.status. However, we will stick with the latter for clarity.

Writing to the Status Bar
You can assign values to window.status via buttons, links, and every other event that is triggered by the user. For example, to have a message displayed when the user clicks a button you can use the following form:
<HTML> <HEAD> <TITLE>status bar</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function showMessage() { window.status = "Fascinating!" }

Example 16-1 (ex16-1.htm). You can display a string in the status bar as a reaction to a user-initiated event such as a button click. You can see the output of this script in Figure 16-1.

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Figure 16-1. Displaying text in the status bar.

Many web pages have begun to implement a technique that displays a short description in the status bar rather than the URL when you place the mouse’s pointer over a link. A description is usually much more meaningful than a URL. The event handler that is triggered when the user places the mouse’s pointer over a link is onMouseOver. The event handler should always return the Boolean value true after the message is placed in the status bar. The only correct way to assign a message corresponding to a link in the status bar is to use the following syntax for the link:
<A HREF="URL" onMouseOver="window.status = 'any string'; return true">

Don’t forget to return true—it’s essential. The following statement creates a link to Wordware Publishing’s site and displays a short message when you place the pointer over it:

Example 16-2 (ex16-2.htm). A script that displays the string in the status bar for 2.5 seconds.

The output for this script is shown in Figure 16-2.

Figure 16-2. Holding the mouse over the WordWare link will display a string in the status bar for 2.5 seconds.

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The setTimeout() method is a possible replacement for the need to return true. This script is very simple. When the user places the pointer over the link, the function showMessage() is called with the desired string. The string is assigned to the status property, and the function clearMessage() is called to clear the status bar after 2.5 seconds, by replacing the current message with an empty string. A much better way to erase a string written by onMouseOver from the status bar is to implement the onMouseOut event handler. This event handler serves as an attribute of a link or client-side image map area, and it is triggered when the mouse pointer leaves an area or link from inside that area (see Chapter 21, Links, Anchors, and Image Maps). This outside movement enables us to assign an empty string to the status bar when the user removes the pointer from the link. Here is another example:
<A HREF="http://www.wordware.com" onMouseOver="window.status = 'Get another cool computer book from Wordware'; return true" onMouseOut="window. status = ''; return true">WordWare</A>

Setting a Default Value for the Status Bar
You have seen that it is possible to write one-line strings to the status bar via the status property. It is also possible to set a default value for the status bar. This value is kept in the status bar as long as no value is written to it by the status property. The general syntax of this property is window.defaultStatus or defaultStatus. When using this property with an event handler, you must return true. Take a look at the following example:
<HTML> <HEAD> <TITLE>status bar</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function statusSetter() { window.defaultStatus = "Click the link for the Wordware Publishing homepage" window.status = "Wordware homepage" } // --> </SCRIPT> </HEAD> <BODY> <A HREF="http://www.wordware.com" onMouseOver = "statusSetter(); return true">WordWare</A> </BODY> </HTML>

When you load the page in the browser, you see a link. At that moment, the status bar does not contain anything (except perhaps a string to report that the page has been

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loaded). When you place the pointer over the link, the message “Wordware homepage” is written to the status bar. When the pointer is taken off the link, the message is replaced by another string—“Click the link for the Wordware Publishing homepage.” This is a nice alternative to using the onMouseOut event handler.

Banners
Banners are classic JavaScript scripts. They were and still are an attractive addition to any web page. Originally banners appeared in the status bar, scrolling several messages in sequence. Some prefer to place the banner in a text box, because they feel that status bar-based banners are annoying to the common surfer. Banners are sometimes very unsteady; their speed is not uniform and they sometimes blink. There is no workaround to make them better. But, all in all, they are still a lot of fun. In this section we take a look at a few different banner-like scripts, most of which were created exclusively for this book.

T-banner
The T-banner simulates a typewriter. It displays each message by typing it in, one character at a time. It appears as if someone is typing the message at a certain speed and deleting it upon completion. First, take a look at the script:
<HTML> <HEAD> <TITLE>T-Banner</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-// set speed of banner (pause in milliseconds between characters) var speed = 100 // decrease value to increase speed (must be positive) // set pause between completion of message and beginning of following one var pause = 1000 // increase value to increase pause // set initial values var timerID = null var bannerRunning = false // create global array var ar = new Array() // assign the strings to the array's elements ar[0] = "Welcome to this JavaScript page" ar[1] = "I hope you enjoy the T-Banner script" ar[2] = "It’s kind of a cool banner" ar[3] = "Don't forget to check out my other scripts" // set index of first message to be displayed first var currentMessage = 0 // set index of last character to be displayed first var offset = 0 // stop the banner if it is currently running function stopBanner() { // if banner is currently running if (bannerRunning)

Global Statements
// set speed of banner (pause in milliseconds between characters) var speed = 100 // decrease value to increase speed (must be positive) // set pause between completion of message and beginning of following one var pause = 1000 // increase value to increase pause // set initial values var timerID = null var bannerRunning = false // create global array var ar = new Array() // assign the strings to the array's elements ar[0] = "Welcome to this JavaScript page" ar[1] = "I hope you enjoy the T-Banner script" ar[2] = "It is designed to be more stable than regular banners" ar[3] = "Don't forget to check out our other scripts" // set index of first message to be displayed first var currentMessage = 0 // set index of last character to be displayed first var offset = 0

At first, the speed of the banner is set to 100. This is equal to the pause between each character of a message in milliseconds. The pause (in milliseconds) between the completion of a message (string) and its deletion is assigned to the variable pause. The identifier for the current timeout is assigned null, because no timeout is set yet. The current state of the banner (false because it is not running yet) is assigned to the variable bannerRunning. An array is then created to hold the strings that are to be displayed as banner messages. The first string is assigned to the first element of the array, ar[0], and so on. The number 0 is assigned to currentMessage because the first message displayed is ar[0]. The index of the last character displayed at a given moment in the status bar is assigned to the global variable offset. It is set to zero because the first appearance of the banner consists of only one character—the first one—whose index is zero. stopBanner()
// start the banner function startBanner() { // make sure the banner is stopped stopBanner() // start the banner from the current position showBanner() }

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This function is called to stop the banner. If the banner is running, the current timeout is cleared. The variable bannerRunning is set to false because the banner is stopped. startBanner()
// start the banner function startBanner() { // make sure the banner is stopped stopBanner() // start the banner from the current position showBanner() }

This function calls the stopBanner function to make sure the banner is stopped, and then calls the function showBanner to start running the T-banner. showBanner()
// type in the current message function showBanner() { // assign current message to variable var text = ar[currentMessage] // if current message has not finished being displayed if (offset < text.length) { // if last character of current message is a space if (text.charAt(offset) == " ") // skip the current character offset++ // assign the up-to-date to-be-displayed substring // second argument of method accepts index of last char plus one var partialMessage = text.substring(0, offset + 1) // display partial message in status bar window.status = partialMessage // increment index of last character to be displayed offset++ // recursive call after specified time timerID = setTimeout("showBanner()", speed) // banner is running bannerRunning = true } else { // reset offset offset = 0 // increment subscript (index) of current message currentMessage++ // if subscript of current message is out of range if (currentMessage == ar.length) // wrap around (start from beginning) currentMessage = 0 // recursive call after specified time timerID = setTimeout("showBanner()", pause) // banner is running bannerRunning = true

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} }

The current message is assigned to the local variable text. The function then continues in one of two directions. The first path is selected if the current message is still being displayed; that is, if the index of the last character that was displayed of the current message is the last character or less. In that case the expression offset < text.length evaluates to true. If the last character to be displayed during this pass through the function is a space, the value of offset is incremented, and no time is wasted on typing the space character. The substring that needs to be displayed during the current iteration is assigned to the local variable partialMessage. Since the second argument of the substring method is the index of the last character plus one, it is set to offset + 1. The current substring is displayed in the status bar, and the value of offset is incremented. The function is called once more after speed milliseconds. When the end of the current message has been reached, another execution path is taken. In this case the variable offset is assigned zero, the index of the first character of a string. To allow posting of the next message in the array, the index of the array element holding the current message is incremented. If the new value of currentMessage is out of range, it is set to zero, so the following message is the first one. The function is called recursively after pause milliseconds. This time the function should take the first route, because the message is only at its beginning. Event Handlers The startBanner function is called when the document has finished loading by the onLoad event handler.

R-banner
While T stands for typewriter, R probably stands for random, denoting the flavor of this special banner. The messages appear by popping up various characters of the message in a random order. Another special effect with this banner is the scrolling motion from right to left. This effect is achieved by simply letting each character that pops up take a three-character space. Take a look at the script:
<HTML> <HEAD> <TITLE>R-Banner</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-// set speed of banner (pause in milliseconds between addition of new character) var speed = 10 // decrease value to increase speed (must be positive) // set pause between completion of message and beginning of following one var pause = 1500 // increase value to increase pause // set initial values var timerID = null var bannerRunning = false // create array var ar = new Array() // assign the strings to the array's elements

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ar[0] = "Welcome to my JavaScript page" ar[1] = "I hope you enjoy the R-Banner script" ar[2] = "It is pretty cool" ar[3] = "Don't forget to check out my other scripts" // assign index of current message var message = 0 // empty string initialization var state = "" // no value is currently being displayed clearState() // stop the banner if it is currently running function stopBanner() { // if banner is currently running if (bannerRunning) // stop the banner clearTimeout(timerID) // banner is now stopped bannerRunning = false } // start the banner function startBanner() { // make sure the banner is stopped stopBanner() // start the banner from the current position showBanner() } // assign state a string of "0" characters of the length of the current message function clearState() { // initialize to empty string state = "" // create string of same length containing 0 digits for (var i = 0; i < ar[message].length; ++i) { state += "0" } } // display the current message function showBanner() { // if the current message is done if (getString()) { // increment message message++ // if new message is out of range wrap around to first message if (ar.length <= message) message = 0 // new message is first displayed as empty string clearState()

At first, the number 10 is assigned to the variable speed, representing the pause in milliseconds between the popping up of two consecutive characters in the current message. A longer pause is assigned to the variable pause, representing the number of milliseconds between the completion of the current message and its deletion. A null value is assigned to the global variable timerID and the Boolean value false is assigned to the variable bannerRunning. Predefined messages are stored in an array. The variable message is assigned zero, the index of the first element of the array to be displayed, ar[0]. The second section of the global statement section consists of only two statements, but they are important for understanding the entire script. The first statement in this section assigns an empty string to the global variable state. The clearState() function is called next. It modifies the value of the global variable state, by assigning it n “0” characters, where n is the length of the current message. The variable state is basically constructed of 0s and 1s. Suppose the first character is a 0. That means that the first character of the current message has not been popped up yet. The same

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applies to the second character and all the rest ones. Therefore, the string starts off at all 0s, and the message is finished when all characters are 1s. stopBanner() See explanation of this function in the T-banner section. startBanner() See explanation of this function in the T-banner section. clearState() See above for information regarding this function. getRandom(max) This simply returns an integer between 0 and max – 1 (see explanation of random()) in Chapter 13. getString()
function getString() { // set variable to true (it will stay true unless proven otherwise) var full = true // set variable to false if a free space is found in string (a non-displaying char) for (var j = 0; j < state.length; ++j) { // if character at index j of current message has not been placed in // displayed string if (state.charAt(j) == 0) full = false } // return true immediately if no space found (avoid infinite loop later) if (full) return true // search for random until free space found (broken up via break statement) while (1) { // a random number (between 0 and state.length – 1 ==message.length – 1) var num = getRandom(ar[message].length) // if free space found break infinite loop if (state.charAt(num) == "0") break } // replace the 0 character with 1 character at place found state = state.substring(0, num) + "1" + state.substring(num + 1, state.length) // return false because the string was not full (free space was found) return false }

At first, the variable full is initialized to true. An infinite loop (terminated by the break statement) is employed to go over all 0 and 1 characters of the state string. If a 0 character is found, the variable full is changed to false. It is not mandatory to

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break up the loop (as done above in getString()) when a free space, or 0, is found, because efficiency is not a concern in a random banner. The remaining part of the function is executed only if a free space is available. An infinite loop generates a new random index on every iteration and checks if the space at that index is taken up. The loop continues to execute as long as the space at the random index is taken up. When a free space is finally found, the loop breaks up, returning the index num of the free space. The value of state is updated by replacing the 0 character at index num with a 1 character. The function returns false upon termination, indicating that the message was not completed. showBanner() As in the T-banner script, this is the main function. The function getString() is called to update the value of state and to determine if the message has been completed. If it has, the current message is updated to the next one by incrementing the value of message, representing the index of the array’s element where the current message is. If the new value is out of range, it is reset to the initial zero value. The function clearState() is called to set up the variable state, as explained above. The function is then called recursively after pause milliseconds. In an ordinary case in which the message is not complete yet, the special display effects are generated. An empty string is assigned to the local variable str, ready to accumulate characters for display. A for loop iterates through the characters of the string ar[message]. If there is a 1 at the same index of the string state, the character from ar[message] is appended to the end of str. An alternative string is appended if a 0 character is found in the string state. The only way to create a right-to-left scrolling effect is to use a fairly long alternative string of a few spaces. Since a space has a very small horizontal span (characters are not uniform in width), a single-space alternative string will create a friendly left-to-right movement! Using an empty alternative string squeezes the string and creates a different right-to-left movement. The built-up string is placed in the status bar, and the function is called recursively after a pause of speed milliseconds. Event Handlers When the document is completely loaded, the startBanner() function is called by the onLoad event handler.

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N-banner
The N(normal)-banner scrolls from right to left in the status bar. You have probably seen hundreds of these banners on the web. Here is a script to implement such a banner:
<HTML> <HEAD> <TITLE>N-Banner</TITLE>

A major advantage of this banner is that it does not include any global statements and only one function. scrollBanner(seed) The banner’s speed and messages are specified as in the previous banners. A singlespace string is assigned to the variable str. It pads the left side of the status bar by leaving a space between the border and the far left side of the message. Since this banner scrolls the messages one after the other, all messages are combined to a single string via the join() method of the Array object. The function accepts one argument, which is assigned to the parameter seed. The value of seed determines the message’s distance from the left side of the status bar. Space characters are used as spaceholders. It is common to start the banner 100 spaces from the left, so the function is initially called with the value of 100. The function chooses one of three routes. The first is selected if the value of seed is positive; that is, if the message has not reached the left panel of the status bar. In this case, a loop is used to concatenate a string of seed space characters to the variable str. The entire message is then concatenated to str. Note that if a string placed in the status bar exceeds the maximum length of the bar, the excess characters are not displayed. After concatenating the entire message to the accumulative string, the value of seed decrements, creating a scrolling effect on the next iteration. A recursive call is constructed in a string in the following form:
var cmd = "scrollBanner(" + seed + ")"

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Suppose the value of seed is 50. The value of cmd is then "scrollBanner(50)". A literal is used here because local variables or parameters cannot be evaluated by setTimeout as arguments of a function. The accumulative string is then placed in the status bar, and the function is called recursively, following a pause of speed milliseconds. The second route is taken if part of the message has been dropped to the left, but some of it still remains in the status bar. This state is checked via the following expression:
–seed < total.length

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If the absolute value of seed exceeds the message’s length, this expression evaluates to false. It means that the length of the message that has gone out of range to the left is greater or equal to the length of the message, meaning that no part of the message actually remains in the status bar. If this expression evaluates to true, only part of the message has gone past the left barrier. If the expression above evaluates to true, the substring total.substring(–seed, total.length) is concatenated to the variable str. This substring represents the part of the message that has not gone overboard (|seed| is equal to the number of characters disposed). The remaining part of the command block is identical to the one used when the first route of the function is taken. The third route that can be taken by the function is the most simple. It places the value of str, a single-space string, in the status bar. It then calls the function recursively with the initial argument of 100.

Summary
This chapter introduced status bar programming. We saw how to display link-related messages in the status bar, as well as default values. We emphasized one of JavaScript’s pioneer scripts, the banner, and discussed some unique banners developed exclusively for this book, as well as other common banners. In terms of JavaScript, we have discussed two properties of the window object, status and defaultStatus. Besides enabling fun applications like banners, the status bar can be used as an output device for displaying critical values while debugging the script.

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URLs and JavaScript
A Crash Course in URLs
JavaScript includes a number of properties and methods for use with URLs. Before discussing JavaScript’s support, a general description of URLs is in order. A URL is a Uniform Resource Locator, a standard way to specify the location of an electronic resource. In short, it is a web address, such as http://www.wordware.com. The definition of a URL is derived from concepts introduced by the World Wide Web Global Information Initiative that have been in use since 1990. URLs make Internet resources available to different Internet protocols. When surfing the net, you often run into URLs in your browser’s “location” box. Such URLs usually start with “http:”, but other protocols such as FTP and Gopher are also supported. Even e-mail addresses can be specified as URLs. A URL is a very convenient, succinct way to direct people and applications to a file or other electronic resource.

General URL Syntax
In general, URLs are written as follows: <scheme>:<scheme-specific-part> A URL includes the name of the scheme being used, followed by a colon and a string. scheme refers to the protocol being used. Valid schemes include FTP (File Transfer Protocol) and HTTP (Hypertext Transfer Protocol). The characters supported as schemes are the lowercase letters “a” to “z”, and the characters plus (“+”), period (“.”), and hyphen (“-”). For flexibility, programs should treat uppercase letters as lowercase ones. For example, both “HTTP” and “http” should be accepted. Examples of schemes are “http,” “ftp,” “gopher,” and “news.” The scheme instructs the application or person how to treat that specific resource. Most schemes include two different types of information: n The Internet machine where the resource resides n The full path to that resource

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Such schemes are usually separated from the machine address by two slashes (//), whereas the machine address is separated from the full path via only one slash (/). Therefore, the common format is: scheme://machine.domain/full-path-of-file As an exercise, let’s take a look at a simple URL: http://www.geocities.com/~chuckeasttom/index.htm The URL’s scheme is http, for the Hypertext Transfer Protocol. The Internet address of the machine is www.geocities.com, and the path to the specific file is /~chuckeasttom/index.htm. You will find that the path portion sometimes ends with a slash. This indicates that the path is pointing to a directory rather than a file. In this case, the server returns either a directory listing of all the files or a default file, if one is available. The default filename is either index.html or home.html, but other variants are also used.

URL Schemes
Hypertext Transfer Protocol (HTTP) HTTP is the Internet protocol specifically designed for use with the World Wide Web, and therefore is most often seen by web surfers. Its general syntax is: http://<host>:<port>/<path>?<searchpart> The host is the Internet address of the WWW server, such as www.geocities.com, and the port is the port number to connect to. In most cases the port can be omitted along with the colon delimiter, and it defaults to the standard “80.” The path tells the server which file is requested. The searchpart is very important. It may be used to pass information on to the server, often to an executable CGI script. It can also be referenced by other languages, including JavaScript as you will soon find out. Another frequently used character is the pound sign (#). It is used for referencing a named anchor. Anchors are often used on web pages to enable linking from one section of the page to another. File Transfer Protocol (FTP) FTP is commonly used for distributing and transmitting files over the Internet. Its general syntax is: ftp://<user>:<password>@<host>:<port>/<cwd1>/<cwd2>/.../ <cwdN>/<name>;type=<typecode> When contacting a site providing anonymous login, the user and password may be omitted, including the separating colon and the following @ symbol. The host and port are exactly the same as in the HTTP URL specification. <cwd1>/<cwd2>/... /<cwdN> refers to the series of “change directory” (cd in Unix) commands a client must use to move from the main directory to the directory in which the desired file resides. Since most servers use Unix operating systems, you can print the working (current) directory by typing pwd at the command line. The name is the desired file’s

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full name, as it is recognized by the operating system. The portion ;type=<typecode> allows you to specify the transmission mode (ASCII vs. binary). Most systems are not configured to work properly with this trailing specification, and some are even misled by it. Gopher Protocol (Gopher) The Gopher protocol is not important for JavaScript scripters. Its syntax is very similar to HTTP’s: gopher://<host>:<port>/<gopher-path> Electronic Mail (Mailto) The Mailto URL scheme is different from the previous three schemes in that it does not identify the location of a file but rather someone’s e-mail address. Its syntax differs widely as well: mailto:<account@site> The account@site is the Internet e-mail address of the person you wish to mail to. Most WWW browsers, including the leaders, Navigator and IE, support this scheme when encoded in an HTML document. Usenet News (News) The News URL scheme allows you to refer to Usenet newsgroups or specific articles in such a newsgroup. The syntax is either one of the following: news:<newsgroup-name> news:<message-id> The newsgroup-name is the Usenet’s newsgroup name (e.g., comp.lang.javascript) from which all title articles will be retrieved by the browser (a maximum number may be specified in the browser setup). message-id corresponds to the message-ID of a specific article to obtain. It is found in the article’s header information. Host-Specific File Names (File) The File URL scheme indicates a file which can be obtained by the client machine. The syntax for the File scheme is: file://<host>:<path> The host is the fully qualified domain name of the system, and the path is the hierarchical directory path to the required file. Leave host empty or specify localhost to refer to the client’s local files. JavaScript Statements (javascript) The JavaScript URL scheme is quite different. Its general syntax is: javascript:<valid-javascript-statement-expression-command>

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This scheme evaluates the expression after the colon. If the expression can be evaluated to a string, a new page is opened and the string displayed. If the expression is undefined, a new page does not open. Other Schemes There are several other schemes that are far beyond the scope of this book. Bear in mind that we are dealing with JavaScript, so we only focus on schemes that might be needed in a script.

location Object
The location object represents a complete URL. Therefore, it is a property of the window object. As always, specifying the window object when referring to a property or a method of location is optional. Each property of the location object represents a different portion of the URL. The following syntax of a URL shows the relationship between the object’s properties: protocol//hostname:port pathname search hash The location object belongs to the window containing the JavaScript script. Singleframe pages have only one location object. However, pages that use multiple frames consist of a window.location (location) object for each frame, as well as one for the main frameset document. For example, a page with two frames consists of three URLs, and a location object is available for each. Because window is a default object in scripts, you can use location rather than window.location. However, if you use this object to specify an event handler script, you must specify the full window.location. Due to the scoping of static HTML objects in JavaScript, a call to location without specifying an object name is equivalent to document.location, which is currently a synonym for document.URL.

location Properties
As you know, the location object consists of both properties and methods. The methods were added later, debuting in Navigator 3.0. In this section we take a look at the object’s properties, basing the discussion on the section “A Crash Course in URLs.”

href
The href property is the most popular one in HTML scripting. The identifier HREF stands for “hypertext reference.” This property supplies a string of the entire URL of the calling window object. This property is for both reading and setting. By assigning a string to this property, you can change the URL of the page in the window. For example, if you want to load Wordware’s home page when the user clicks a button, you can use the following syntax:

This statement does not generate an error because “location” is not a reserved word. However, it deletes the location object inside the function in which location is assigned. Since location is not a browser object but rather a simple variable, it is fully accessible from outside the function.

Chapter

You can also retrieve the full URL of the current window (the current window object to be exact) by reading the URL value. For example, if you want the full URL of the current file, not including the filename itself, you can use the following script segment:

Suppose this file is located at http://www.geocities.com. When Netscape Navigator loads a page, it holds the loaded URL in a cell somewhere in memory. Only when another page has begun loading (data is being transferred) is the value of that cell modified to match the new URL. When you read the value of location.href, you are reading the value of that cell. However, when you assign it a string representing another URL, the value held in that cell does not change immediately. Only when the page at the specified URL is found on the server is the value of the cell updated. Microsoft’s browser differs in this case. When the user assigns a string to location.href, it automatically updates the corresponding cell in memory. Therefore, if you display the value of the property location.href immediately after you assign it a value, the assigned value appears. In Navigator, on the other hand, the original URL is still displayed because the file at the new URL has not been found yet. Let’s sum things up. The displayed value of the preceding script on each of the leading browsers is as follows: n Netscape Navigator—http://www.geocities.com n Microsoft Internet Explorer—http://www.microsoft.com Depending on your browser, the value of location.href may be encoded with ASCII equivalents of nonalphanumeric characters. Such characters appear as a percent sign (%) followed by the ASCII code of that character. The most commonly encoded character is the space, %20. You can run such URLs under the unescape() function to convert them to ISO Latin-1 format. Suppose you have an HTML file named foo.html located in a certain directory, better known as a folder on Macs and Windows 95. Loading the full path of this directory in the browser should normally show the listing of all the directory’s files, provided that a default filename supported by the server is not included in that directory. You can

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Chapter An anchor is a mark for other data to point to. It enables you to create a link to a specific place somewhere in a given web page. Suppose you have a web page that provides information on various VCR models of different brands. A user who is looking for specific information on Sony VCRs should not have to scroll through other makes,

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say JVCs. Therefore, you can create a named anchor somewhere near the name Sony. An anchor is created using the following syntax:
<A NAME="sony1">Sony VCRs</A>

The text “Sony VCRs” appears on the page as normal, but it serves as an anchor. You can direct the user to the “Sony VCRs” section via a simple link to the anchor, in the following fashion:
<A HREF="#sony1">Get information on Sony VCRs</A>

In this case, the link’s URL is the anchor’s name preceded by a hash mark (#). When the user clicks on the link “Get information on Sony VCRs,” the browser automatically “scrolls” down to the anchor named “sony1” (you can’t see the scrolling, of course). You can also direct the user to that anchor from within another page. For example, suppose the full URL of the VCR page is http://www.vcr.com/information/index.html. Now, let’s say you want to provide a link from the page http://www.electronics.com/VCRlinks/new.html to the page containing information on VCRs, and, in particular, to the Sony section. You can accomplish this task by including the URL of the VCR file, as well as the Sony anchor name, somewhere in the electronics file:
<A HREF=" http://www.vcr.com/information/index.html#sony1">Get information on Sony VCRs</A>

This form enables you to specify the document URL as well as the specific anchor name. By specifying only the anchor name, it is assumed that the anchor resides in the current document, just as if you specify a filename without a full path, it is assumed to reside in the same directory as the HTML document. Such URL references are known as relative or partial. When you click on a link to a URL containing a named anchor reference, the new URL, or location, consists of the hash mark (#) followed by the anchor name. This portion of the URL is considered a part of the URL, just like the path or filename. After linking to an anchor, user-initiated scrolling does not affect the URL (or the location.href value). JavaScript provides a property for the current anchor reference in the URL. This property is named hash (location.hash), because anchor references are stored in a hash table. You can assign a string to location.hash in order to direct the browser to a specified anchor within the current page. Like the href property, hash is also readable. You should use this property only when dealing with local anchors residing within the current document. Suppose the following document is saved as an .html file on a server or your computer:
<HTML> <HEAD> <TITLE>status bar</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function loadPage() {

The function loadPage() (called via the onLoad event handler) attempts to load the page http://www.geocities.com/~chuckeasttom/index.htm. Since the browser does not wait until the page is loaded, the function continues to execute, advancing to the following statement. However, this statement also attempts to modify the URL of the page by specifying an anchor, via the location.hash property. Since the anchor resides on the current page, it is loaded immediately. The anchor is not found on the current page because it is on the page that was loaded before. By trying to allocate an anchor, the onLoad event handler is triggered, executing the function once again. The encountered loop is obviously infinite, and continues until the user presses the big red Stop button.
function loadPage() { location.href = " http://www.geocities.com/~chuckeasttom/index.htm #certifications" }

When attempting to modify multiple properties of href, you should assign the property href instead, as it refers to a complete URL, including anchor references, search specifications, and all other properties of the location object. In general, the location.hash property is the only one that can be assigned separately to adjust the current anchor referencing position within the current document. Also, bear in mind that a page reload will follow the property assignment. A common problem for beginners is that the value of the hash property seems to be inconsistent at times. For example, location.hash evaluates to an empty string if no anchor is referenced. If an anchor is specified, though, both the hash mark (#) and the anchor name are part of this property. When changing the value of an anchor, do not include the hash mark. The following HTML document will clear up this matter:
<HTML> <HEAD> <TITLE>location.hash property</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function goNext(nextAnchor) { location.hash = nextAnchor } // --> </SCRIPT>

Example 17-1 (ex17-1.htm). A script that jumps to anchors without using common links.

In order to observe the effect of the script in Example 17-1, you must resize the browser window to make it smaller than the full document (the scroll bar should appear), because referencing a named anchor has no effect if the anchor is already in view. By clicking each of the four buttons on the page, the focus is placed on a corresponding anchor and on a different button. Clicking the fourth button scrolls the page to the first anchor and to the beginning of the page. Since HTML does not support linking form buttons to anchors, you must use JavaScript to accomplish such a task.

host
The location.host property is not commonly used, but we shall cover it for completeness. This property represents the <host>:<port> part of a URL, not only the <host> (see location.hostname). When the scheme is file and the file is local, this property is an empty string. When no port is specified, the value of location.host is equal to location.hostname, or <host>. Suppose the complete URL of a page is http://www.geocities.com:80/SiliconValley/9000/index.html. The value of location.host, if queried on that page, is www.geocities.com:80. This value is always equal to the expression location.hostname + ":" + location.port. The colon is included only when a port is explicitly specified. The basic rule is that if the browser’s “location” box does not include the port, it is not part of the location.host string.

If you call this function from the page http://www.geocities.com/~chuckeasttom/index.htm, the value displayed in the alert box is www.geocities.com:80. The same value is displayed if the page is loaded directly via the full host specification: http://www.geocities.com/~chuckeasttom/index.htm.

hostname
The location.hostname property is almost identical to location.host, except that it does not include the port number if specified in the URL. That is, it consists of only the <host> portion of the complete URL (see the section “A Crash Course in URLs”). The location.hostname is simply the Internet address of the hosting machine. This property evaluates to www.geocities.com on the following two URLs: http://www.geocities.com/~chuckeasttom/index.htm http://www.geocities.com:80/~chuckeasttom/index.htm

pathname
The pathname component of the URL consists of a directory structure, relative to the hosting server’s root volume. In terms of http, this is the <path> portion of the URL. If the file is located in the root directory of the server, the pathname property evaluates to a single slash (/), followed by the complete filename. The pathname property always includes the name of the file where the script is located. This property returns a nonstandard value in Internet Explorer. When the file is on the client’s computer, backslashes are used in place of slashes to separate directory names in the path. For example, if the complete URL of the hosting document is http://www.geocities.com/~chuckeasttom/index.htm, the value of location.pathname is "/~chuckeasttom/index.html".

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port
As expected, the location.port property holds the port number, or the <port> portion of the general URL syntax. These days, few web sites require an explicit specification of the port number as part of their URL. When a port is not specified, it defaults to the standard 80, which is not part of the location.port property. If you intend to construct a URL from the values of the port and the hostname properties, remember to separate them with a colon.

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protocol
The protocol component of a URL is more commonly known as the scheme (<scheme>). The location.protocol property holds the scheme component of the document’s URL, followed by a colon. The scheme component should normally be “http:”, but other schemes are also supported. For more information on the most popular schemes, see the section “A Crash Course in URLs” at the beginning of this chapter. You can display the mocha: or javascript: protocols by loading one of them as the URL of the document and then typing alert(location.protocol). You can also try loading the following strings as URLs instead: n javascript:alert(location.protocol) n mocha:alert(location.protocol)

search
When you submit a form, you sometimes find that the URL of the retrieved document is followed by a question mark (?) and an encoded string. For example, a typical search on Yahoo looks like http://search.yahoo.com/bin/search?p=perl+book&a=n. The value of location.search is precisely that, including the question mark. Each part of the search specification (?p=perl+book&a=n) is usually delimited by an ampersand (&), as seen in the above string. Nonalphanumeric characters are encoded and replaced by their corresponding two-digit ASCII code, preceded by a percent sign (%). If text fields or text areas consist of space characters, they are replaced by plus operators (+). The string following the question mark is known as the search query, although it does not serve as a query submitted to a search engine. First of all, remember that if a search query is specified (including the question mark), the URL of the page is actually the string preceding the query. For example, you can load Intel’s home page via the URL http://www.intel.com/index.htm?Intel+home+ page rather than the standard URL for that page, http://www.intel.com/index.htm. Since Intel’s page does not use any search queries, a query specification is extraneous. You can load every page on the web by entering its regular URL followed by any search query. This feature enables the usage of search queries in JavaScript. For example, you can prompt the user for his or her name, and pass it on to all other pages on your site as a search query. Along with cookies (explained in Chapter 22, Implementing Cookies), the location.search property serves as a way to store permanent data acquired from an outside resource. The following example consists of two pages, page1.html and page2.html:
<HTML> <HEAD> <TITLE>User first name input</TITLE> </HEAD> <BODY> <SCRIPT LANGUAGE="JavaScript"> <!-var usernm = prompt("Enter your first name:", "")

The script on page1.html prompts the user for his or her first name, and then, using a search query preceded by a question mark, loads page2.html with the input first name. The script on page2.html calls a function that strips off the question mark, returns the bare query, and then displays the user’s first name. The location.search property serves as a convenient way to pass small pieces of information between documents, as shown in this example. Example 17-2 demonstrates the use of this property in a more complex script:
<HTML> <HEAD> <TITLE>Matches game</TITLE> </HEAD> <BODY> <SCRIPT LANGUAGE="JavaScript"> <!-// return the current search query excluding question mark function stripQuery() { // assign value of location.search and length to local variables var search = location.search var length = search.length // if no query specified if (search == "") // return number of matches at beginning of game return "25"

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// strip question mark off string var query = search.substring(1, length) // return the stripped-off query return query } // print the desired row of match images function placeMatches(num) { // place num matches via loop for (var i = 1; i <= num; ++i) { document.write('<A HREF="' + getURL(i, num) + '"> <IMG SRC="match.gif" BORDER=0></A>') } } // return computed URL for match image link function getURL(pos, num) { // assign position of match in row from right var distance = num – pos + 1 // e.g., 19th match among 20 in row, 20–19+1 = 2nd match from right // if the match is not one of the last three in row if (distance > 3) // link does not do anything return "javascript:alert('Choose one of last three matches')" // else not required because return statement terminates function // return number of matches needed in following load of page return "ex17-2.htm?" + (num – 4) } // number of matches to be displayed var num = parseInt(stripQuery()) // assign instructions to variable var instructions = "" instructions += "The objective of the game is to force the other player " instructions += "(the computer) to pick up the last match. On each turn " instructions += "you may pick up some matches, but only 1, 2, or 3. The " instructions += "computer does the exact same on its turn. Play smart, " instructions += "or else you will be devastated." // if no query specified if (num == 25) // display instructions alert(instructions) // if only one match remains if (num == 1) { // print the match image and a link to enable a new game document.write('<IMG SRC="match.gif"><BR><BR><A HREF="ex17-2.htm">Play again?</A>') // tell the human he / she lost alert("I win -- come back when you improve!") } else // impossible condition if (num < 1)

Let’s explain the rules of the game in Example 17-2. The game starts when the user first loads the page and 25 matches are displayed in a row. The user must pick up one, two, or three matches by clicking on the corresponding match. For example, to pick up only one match, the user must click on the far-right match. Clicking on the second match from the right is equivalent to picking up two matches, and the same applies to three. After the user picks up some matches, the computer plays its turn, following the same rules as the user. The objective of the game is to force the other player to pick up the last match. That is, the one who picks up the last match loses the game. This game was designed to prove that computers are smarter than humans, because you cannot win. The real reason that it is impossible to defeat the computer is that the user makes the first move. The number of matches selected by the computer is equal to four minus the number of matches selected by the user. For example, if the user picked up two matches, the computer also picks up two. If the user selected three matches, the computer goes with one. Finally, if the user selects one match, the computer selects three. Therefore, each dual move (user and computer sequence) ends up with four matches lifted. After six dual moves, 24 (4 * 6) matches have been removed, and only one remains. It is then the user’s turn, which means that the computer records another victory. The stripQuery() function is very simple; it returns the search query without the question mark. If no query is specified, then the user has not begun the game, and the returned string is defaulted to "25". The placeMatches() function is also very simple. It accepts the number of matches to be printed and uses a loop to print the corresponding number of match.gif images. Each image is also a hypertext link, where the specified URL is retrieved by the getURL() function, based on the index of the current match (a positive integer i) and the total number of matches that are placed (num). The function getURL() accepts the index of a given match as well as the total number of matches to be placed. The match’s index corresponding to the far-right match is assigned to the variable distance. For example, if there are 25 matches, and the value of pos is 23, the value assigned to distance is 25 – 23 + 1 = 3. If the assigned value is greater than three, the data of the given match indicates that it is not one of the last three matches in the row, so the returned URL is a JavaScript alert statement (using the javascript: scheme, or protocol). The function is terminated if the given match is not one of the last three, so the remaining portion applies only to matches that are

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one of the last three in the row. In this case, the returned URL is the bare URL of the document with a search query equal to the remaining number of matches (the current number of matches minus four, because the computer always completes the user’s move to four—see the explanation of algorithm above). The global statements are also a very important part of the script. At first, the query of the current page is converted from a numeric string to a number and then assigned to the variable num. If the value of num is 25, the game has just begun, and the instructions are displayed in the form of an alert box. If the value of num is 1, the game is over (the computer wins), and a corresponding message is displayed, followed by a link to restart the game by loading the current document without a query. If the value of num is less than 1, an impossible state has been encountered, meaning that the user has tried to cheat or override the script by modifying the search query and an alert box reports this finding. The game is currently under way for all other values of num, so the function placeMatches() is called to place the matches according to the given situation.

location Methods
The location object (window.location) also has several methods. They are supported by both Netscape and Microsoft’s latest browsers.

reload
The location.reload method forces a reload of the window’s current document. Its general syntax is:
location.reload([true])

Specifying the Boolean value true as the method’s optional argument forces an unconditional HTTP get of the document from the server. An unconditional get retrieves a document directly from the server, ignoring the content of the browser’s cache, which might already contain the desired data from that document. Therefore, true should not be specified unless you have reason to believe that either disk or memory cache is broken, or the server has a new version of the document. If such a version is available, you must force an unconditional HTTP get because the version of the document stored in cache is different from the new version on the server. This situation is common to CGI-generated documents. The reload() method simply reloads the document its URL stored in location.href. It uses the same policy as the Reload or Refresh button. Microsoft has opted to label the button “refresh” rather than “reload,” but will probably keep the same method names. The exact reload process depends on the cache handling menu option. In Netscape Navigator, the user sets the default value of this process by choosing Network Preferences from the Options menu, and specifying Verify Documents on the Cache tab of the Preferences dialog box.

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The reload() method does not force a transaction with the server under normal conditions. However, if the user has set the preference to “Every Time,” the request is an unconditional get using an “if-modified-since” HTTP header. HTTP headers are passed to the browser when a document is retrieved from the server. It contains important information regarding the current document. If the user sets the preference to “Every Time,” the browser checks the transmitted HTTP header to see if the document has been updated according to the “last-modified time” property. If it has, the document cannot be loaded from the cache which holds a previous version of the file. In short, reload() will bring the cache’s version unless the user has specified “Every Time” and the document has changed on the server since the last time it was loaded and saved in the cache. Since its size is limited, the cache might lose a document version that has not been reloaded for a long time. In this case, the document needs to be fully loaded from the server, even if it has not been changed since the previous load. In event handlers, you must specify window.location.reload() instead of simply using location.reload(). Due to the static objects’ scoping in JavaScript, a call to location without specifying an object name is equivalent to a call to document.location, which is a synonym for document.URL. This concept is explained later in the chapter in greater detail. You have probably experienced situations in which you leave your computer connected to a host and go out for a break, then come back to find that the connection has been dumped. The usual cause is that the host (server) has disconnected you because you have not transmitted any data via the server for a long time. You can overcome this problem by periodically reloading a JavaScript document from the server. Next time you go out for lunch, load the following document in the browser’s window:
<HTML> <HEAD> <TITLE>stay connected</TITLE> </HEAD> <!-- 200000 milliseconds == 200 seconds --> <BODY onLoad="timerID = setTimeout('window.location.reload(true)', 200000)"> </BODY> </HTML>

The onLoad event handler is used to call the reload() method. A setTimeout() method delays the reload procedure for 200,000 milliseconds, or 200 seconds, from the moment the document is completely loaded. Since it is used in the form of an event handler, the reload() method must be fully specified, including the window object reference. The true argument forces the transaction with the server.

replace
The replace() method is also a property of the location, or window.location, object. It overwrites the current history entry with the specified URL. The current history entry is the most recent URL added to the history list or the URL of the previously loaded page. This is the URL that is retrieved when the user presses Back,

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provided that the Forward button is dimmed out. The general syntax of the replace() method is:
location.replace("URL")

After the replace() method is used, the user cannot navigate to the previous URL via the Back button. Once again, bear in mind that event handlers require a full method specification. Suppose you want the user to load page B by clicking a link on page A. Instead of using a plain hypertext link, you can invoke this method to load page B (using the URL of page B as the argument). Once page B has loaded, the user cannot return to page A via the Back button.

Another Location—document.location (document.URL)
So far, any reference to the location object defaulted to window.location. But, there is another location in JavaScript—document.location. To avoid confusion, Netscape decided to change document.location to document.URL. However, Netscape 3.0 and up support both. Microsoft’s Internet Explorer 3.0 and later versions support only document.location. In this chapter, we use document.URL. The property document.URL holds the complete URL of the current document. In contrast to window.location(.href), it is a read-only value. It does not know windows from frames; it only recognizes the document that contains the script and the reference to this property. Keep in mind that the URL belongs to the document, not the window. Therefore, when a window consists of multiple documents including frame structures, a single frame’s document.URL is different from window.location(.href), from any other frame’s document.URL, and from the main frame set’s document.URL. If you want to load a new document into the browser’s window, you have to use the write-enabled window.location (==window.location.href). If you want the URL of a specific frame, you should use document.URL. You can also use this property to retrieve the URL of the window if the document does not have frames and you are sure you do not want to change the URL (to load another page). Since document.location is still in use (especially with Internet Explorer 3.0), you must be very careful when using the location property. If you are not a very experienced scripter or do not fully understand object scoping in JavaScript, it is a good practice to always specify the calling object, either window or document. When you refer to location in a script, it defaults to window.location, because window is the default object inside scripts. However, when you specify the bare location in an event handler script, the calling object defaults to document; that is, location defaults to document.location.

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Search Utilities
You may have noticed that multiengine search utilities are beginning to rule the web. For example, you can use one form to search Infoseek, AltaVista, and Yahoo. There are basically two ways to create such search interfaces: n Via server-side CGI scripts n Via client-side JavaScript scripts Since CGI is beyond the scope of this book, we are going to discuss just the second method. JavaScript is a very flexible cross-platform language. You can perform a specific task with many completely different scripts. For example, you can put a long script in your page to enable a multiengine search interface. You can also place a form in the page to submit the query to another page that contains the script. You can even call a function located in an external script to do the work. You have probably been exposed to advanced HTML for quite a while, so you should know how forms are submitted. There are generally two submission methods: n get n post The get method calls a specified file along with the content of the form’s fields. The ACTION attribute specifies the name of the document or script to be called when the form is submitted. Take a look at the following form:
<FORM METHOD="get" ACTION="file1.html"> <INPUT TYPE="text" SIZE=50 NAME="userid"> <INPUT TYPE="text" SIZE=30 NAME="passwd"> <INPUT TYPE="submit" VALUE="submit form"> </FORM>

This construct creates a form with three elements. The first two are simple text boxes, or fields. The third is a submit button, which triggers the submission. That is, when the user clicks the button, the form is submitted according to the ACTION of the METHOD. Suppose the user enters “input of first box” in the first field, and “input of second box” in the second field, and then clicks the Submit form button. The form is submitted. In this case the method is get, so the browser “gets” the specified file, file1.html. Nevertheless, the file, including a search query, is retrieved. The full URL retrieved by the browser in this case is file1.html?userid=input+of+ first+box&passwd=input+of+second+box. Notice that each value in the search query is separated by an ampersand. In addition, the value entered by the user in each field, or the element’s value in general, is preceded by the element’s name followed by an equal sign (=). The constructed URL is loaded, and the search query can be used if the loaded file is an HTML document with a JavaScript script. Now let’s take a look at an actual example:
<HTML> <HEAD> <TITLE>Multiple engine search</TITLE> </HEAD>

Example 17-3a (ex17-3a.htm). The search interface can be added to any page.

This form is a bit more complex than the previous one we have explained. It consists of two value-contributing elements, a field (text box) and a SELECT object, enabling the user to choose an option from a list. The value of the selected OPTION is the contributed value of the SELECT element. Take a look at the following image which demonstrates a possible user input:

Figure 17-3. Possible user input for the interface created in Example 17-3a.

For the output demonstrated in Figure 17-3, the loaded URL is ex17-3b.htm?query= JavaScript&engine=altavista. The name of the text box element is “query,” which

Example 17-3b (ex17-3b.htm). The script that interprets the user’s input and calls the appropriate search engine.

Figure 17-4. Calling the appropriate search engine.

The script consists of only two functions. The first one, prefixObject(), is a constructor. It creates an object whose properties are the query prefixes for the supported search engines. A query prefix is the URL by which a query can be submitted to a search engine. The prefix is followed by the encoded inquiry (e.g., the user’s keywords). For example, you can look up the keyword “JavaScript” in Infoseek by loading http://guide-p.infoseek.com//Titles?qt=JavaScript. Each search engine has its own unique prefix, so the prefix for each of the supported engines must be explicitly specified. The constructor function prefixObject() assigns each prefix to a property, named according to the search engine with which the prefix is associated. For example, Infoseek’s prefix is assigned to a property named infoseek in the following fashion:
this.infoseek = "http://guide-p.infoseek.com//Titles?qt="

If you know the prefixes, you can easily extend the script to support additional search engines. It is not difficult to find such a prefix—just run a normal search on the desired engine and then extract the desired prefix. Since prefixes are sufficient for most

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engines, suffixes are only occasionally used. Search engines’ prefixes are subject to change and should be maintained by the script owner (webmaster). The global variable prefix is an instance of this object, so its properties are the search engines’ prefixes. The callSearch() function is also very simple. At first, it assigns the encoded user input (keywords) to the variable query. It also assigns the selected search engine to the variable engine. The expression prefix[engine] is equal to the selected search engine’s prefix, because the values of the form’s options (see Example 17-3a) are equivalent to the names of the properties used in this script. The expressions stored in prefix[engine] and query are combined to construct the full desired URL. The combined string is then loaded as the new URL via assignment to the location.href property. You have probably noticed that the function is not called as an immediate script. It is called via the onLoad event handler, which delays the execution until the page has finished loading. In this case, a setTimeout statement is used to delay the execution another three seconds, giving the user a chance to terminate the process. This is extremely important, especially if the user is surfing in “reverse” using the Back button. A form consisting of a single button is used to clear the timeout via the clearTimeout() method, which is handed the identifier of the initial timeout. The user can click this button to terminate the search process before the specified search engine is actually called.

Summary
In this chapter I discussed JavaScript’s URLs. First, I introduced common URL terms, including the various supported schemes (http, ftp, gopher, etc.). I then presented the window.location object with all its properties and methods. The href property is used to link documents to each other. I also discussed a URL-processing- based multiple-engine search utility and introduced another URL-related element, the document.URL property, also known as document.location. You should be mastering URL handling by now, because we will move on to more advanced URL-related concepts, such as frames, later in the book.

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Using the History List
What is a History List?
As you surf the web, you will load many different pages, each with its own URL. The browser maintains a list of the most recent URLs, which can be viewed with ease in both Navigator and Internet Explorer. The history list behaves like a LIFO (Last In First Out) queue, where the Back button climbs up the list so URLs loaded by the Back button are not entered into the history list. Therefore, the history list does not always contain all the recently visited pages. For example, if you reach a page named “a.html” and you press the Back button to load a page named “b.html,” its URL replaces the URL of “a.html.”

The history Object
The history list is represented in JavaScript by the window.history object. This object lets you deal with the history list but not with its exact data. That is, actual URLs maintained in that list cannot be extracted or otherwise modified by a script. The only property of this object is its length. Its methods enable you to load the list’s entries but not to manipulate the URL explicitly. You can take advantage of this object to automatically navigate the user’s browser backwards. Another possible application is to create the equivalent of the browser’s Back button directly in the document. Since the history object is a property of the topmost window object, you have the option to refer to it as window.history or simply history.

History List Length
You can access the number of entries in the history list via the history.length property. It works exactly like the length property for string and array objects. You can use this property to find how many pages the user has visited lately:

This script displays an alert message which depends on the number of entries in the history list.

History List Entry Indexing
As with arrays, each entry of the history list has an index, which differentiates it from the other elements of the list. However, the indexing method is quite different from character indexing in strings or element indexing in arrays. As opposed to these indexing algorithms, the history list indexing scheme does not feature a minimum value. The index of the document currently loaded into the browser’s window is 0. The index of the document that was loaded before the current document, the one that can be reached by pressing the Back button, is –1. The document before that is indexed at –2, and so on. Similarly, documents that were first loaded after the current document are indexed positively. The index of the first document loaded after the current one, the one that can be retrieved via the Forward button, is 1. The following one is indexed at 2, and so on. The complete index resembles an axis with no limit at both ends. The history list is dynamic (changes rapidly) because whenever the page in the browser’s window is replaced by a new document, the current entry becomes the previous one, and a new document takes its place. The desired shifting in terms of indexing is performed automatically by the browser, so you don’t have to worry about it. Since most people tend to surf different places at different times, the content of the history list almost never repeats itself. You might think that by creating a very structured site, you can control the way the user surfs your site and thus be able to forecast the content of the history list. This is generally impossible, and you should not even try to do it.

history Methods
You can implement the history object’s methods in your script to enable the user to navigate among the list’s URLs. You cannot access the string value of any URL, but you can load any of them into the browser’s window.

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back
The back method performs the same action as the Back button in the browser’s toolbar. It loads the most recent entry in the history list—the entry with index –1. The following HTML tags can be used to create a Back button in a web page:

forward
The history.forward method is equivalent to the Forward button in the browser’s toolbar. It loads the entry with index 1. It is less useful than the preceding method because the current document is usually the most recent in the list, so there is no URL that can be loaded when this method is invoked. You must take special precautions when using this method, because it normally does not have any effect. It should be used only when you are sure that you have full control over the user’s navigational path. The following sequence creates a Forward button for a web page:
<FORM> <INPUT TYPE="button" VALUE="Forward" onClick="history.forward()"> </FORM>

go
The go method is also one of those less useful methods featured in JavaScript. It enables you to load a history list entry in the browser’s window. You must have full control over the user’s navigating path in order to implement this method for useful purposes. This method accepts one argument. The most basic argument is the index of the history list that you want to retrieve. This can be any integer number that has a corresponding history list entry. If the argument is 0, the current page is loaded, or better said, reloaded. For example, the following call is equivalent to invoking the history.back() method:
history.go(–1)

When you want to jump back to the entry with index –1, use history.go(–1) rather than history.back(), because, among other reasons, you can just change the argument in order to jump back a few steps instead of only one. The same applies to history.forward(), which is equivalent to the following call:
history.go(1)

Also bear in mind that this method does not return any value but causes immediate navigation. Alternatively, you can specify one of the URLs as the argument of this method. A portion of the desired URL is also sufficient, provided that it is a unique substring of only one entry. In both cases, the specified string (literal or value) is compared against all entries, and the one whose URL includes the specified substring will be loaded. Unfortunately, you cannot extract the URL, just load it. The following script segment retrieves Netscape’s home page (www.netscape.com or home.netscape.com) if it is resident in the history list:

In version 3.0 and up, Netscape Navigator provides an explicit method for this: location.reload().

Security Aspects of the history Object
It would be very useful to be able to extract and process URLs that reside in the history list. For security reasons, this functionality has been excluded thus far. First of all, nobody should use the back door to know where the user has been and, secondly, data can be easily submitted from the client to the server via e-mail. Netscape has solved the e-mail breach of security by displaying a warning whenever an e-mail is sent (other than that sent explicitly by the user from the mail window). Netscape’s solution is not foolproof since the user might have disabled this warning, might not pay attention, or might ignore it altogether. The problem with the history list’s entries is that they contain complete URLs. A URL may contain extremely confidential information, especially in the form of a search query. For example, the user might have recently submitted a form with a field containing a credit card number. The form may have loaded another page with a search query containing the credit card number. Thus, credit card numbers or other secure information may be revealed and gleaned by malicious individuals.

Summary
This chapter focused on JavaScript’s window.history object. Since this object is not important and does not have many uses, we kept the discussion short. The most important property of the object is the go method. Besides having a unique functionality of its own, the go method can replace all other history methods. Because it is closely related to URLs, the history-related function replace was discussed in the previous chapter. The following chapters describe various concepts of JavaScript’s object model.

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The document Object—Colors, Output, and Properties
The document Object
The document object is a property of the window object. Every window object, whether it is a full browser window, a frame, or a JavaScript-generated window, has a document property. The document object encapsulates everything that exists in the content region of the browser’s window or frame. It is the parent object of the web page’s content, including links, anchors, colors, titles, and so forth. The document object’s properties are spread over the entire HTML document. Some are commonly placed in the head portion, while others are normally found in its body portion. The document object does not consist of any event handlers. You might have thought that onLoad, onUnload, and onError belong to this object, but they are actually properties of the window object. Not all document object properties are HTML-driven content. For example, the lastModified property provides the date on which the document was last modified, even though this date is not provided explicitly in the document but rather in the document’s unseen header. The document object groups many content-related properties, such as text color, background color, and others. It also includes more complex properties which are actually objects, such as forms, links, and image maps.

The title Property
The document’s title is set by default to its complete URL. Most web page authors replace this title with their own text. It is specified in the <TITLE></TITLE> tag pair in the head portion. The title, which usually appears in the title bar, also identifies the browser in the operating system’s environment. When the web page includes frames, the title bar displays the title of the document that contains the frameset. Documents providing the content of the frames may include their own titles, but they don’t affect the title bar.

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The document.title property cannot be set by just any JavaScript script, but only during construction of the entire document. You can use the title property in many different ways. Suppose you want your document to have a large header containing the document’s title. Here is such a page’s outline:
<HTML> <HEAD> <TITLE>Chucks Home Page</TITLE> </HEAD> <BODY> <SCRIPT LANGUAGE="JavaScript"> <!-document.write("<CENTER><H1>" + document.title + "</H1></CENTER>") // --> </SCRIPT> </BODY> </HTML>

Figure 19-1. Using the <TITLE> tag.

If you do not include the <TITLE></TITLE> tag pair, the title defaults to the complete URL of the document, including any search query, if one is specified. If you place the <TITLE></TITLE> tag pair without text in between, the title still defaults to the URL. If the text between the opening and closing tags does not include any characters except one or more space characters, the document is assumed untitled, and document.title remains an empty string, or a space-filled string in the case of Microsoft

Chapter

The document’s title can be scripted via JavaScript as well. Its general reference is document.title. If the document does not make up the main browser window, the reference must be preceded by the specific window to which the document belongs. The title of a frame document is also accessible via JavaScript, using the same procedure.

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Internet Explorer. The property document.title never evaluates to the URL, unless you specifically write the URL between the <TITLE> and </TITLE> tags. Because the value of the title bar also identifies the browser application in the operating system environment, neither Netscape nor Microsoft have made it possible to explicitly modify it. Since it would not make any sense to identify an application as scrolling text, for example, the value of the title bar can be assigned only via the document.title property.

Colors
JavaScript supports several web page color properties, such as background colors and activated link colors. They are all constructed by the same method. There are generally two ways to construct color specifications: n Netscape color names n Hexadecimal triplets The tendency among web page designers is to use Netscape color name specification, which is more intuitive and easier to maintain. For example, you can set the document’s background color in the following fashion:
<BODY BGCOLOR="white">

Before Netscape and Microsoft implemented such color names the only way to specify a color was via hexadecimal triplets. These are still supported, so you can set the background color to white, for instance, in the following way:
<BODY BGCOLOR="#ffffff">

Even though this method is not necessary any longer, you will still see it used frequently so it is a good idea to have a basic understanding of how it works. As you can see, the triplet is constructed of three two-digit hexadecimal numbers. They represent the red, green, and blue elements of the color descriptor. In total, there are approximately 17 million combinations, which is equal to the number of colors supported by a typical Macintosh or SVGA color display. However, Netscape uses a much more limited color cube. The cube consists of all the combinations of 00, 33, 66, 99, CC, and FF for each of the color descriptors. The result is 216 (6 * 6 * 6) distinct colors. The cube occasionally varies. On Macs, it includes the full 256-color palette. On Windows systems, if more than 40 (256 – 216) colors are already in use, the cube is minimized to only 125 (5 * 5 * 5) colors. For now, we shall base our discussion on the standard 216-color cube. Many more colors that are the result of dithering, or mixing, are beyond the standard cube. An HTML document may consist of several color specifications. The following script segment demonstrates them:
<BODY [BGCOLOR="#backgroundColor"] [TEXT="#foregroundColor"]

This script basically prints tables. Each table cell contains a transparent image which defines the size of the cell. Each cell is also assigned a background color which determines the color that fills that cell. When you click on the image, the hexadecimal triplet is displayed via an alert box, and the background color is set to the selected color. The main outline of the cube is a table with one row and six cells (columns). Each cell contains a table of all non-dithered colors with a given blue descriptor. There are six tables in total, one for each of the non-dithered colors: 00, 33, 66, 99, CC, FF. Since there are six non-dithered hexadecimal values, each table is 6 x 6. Each row presents a red hexadecimal value and each column represents a green one. The final output can be viewed in the following screen:

A six-element array is created as an instance of the Array object. The elements of the array are assigned the six values from which the 216 non-dithered colors, which are supported on all platforms, can be combined. The drawCube() function call at the end of the script is also global. display(triplet)
// accept triplet string and display as background color function display(triplet) { // set color as background color document.bgColor = '#' + triplet // display the color hexadecimal triplet alert('Background color is now ' + triplet) }

This function accepts three arguments: the red, green, and blue descriptors. It creates a table cell with the combined triplet as the background color. The content of each cell is a gif89 transparent image, place.gif. The image’s height and width specification determines the size of each cell. Each image is also a hypertext link to a “javascript:”-scheme URL, which calls the display() function. The argument to this function is a hexadecimal triplet which is also used for the cell’s background. Note that all the global array’s descriptors are strings, so the plus sign is used to concatenate the three double-digit hexadecimal values. drawRow(red, blue)
// draw table row based on red and blue descriptors function drawRow(red, blue)

This function accepts the red and blue descriptors and prints a table row. The content of the table is created by six calls to the drawCell() function, passing the red and blue descriptors “as is” and a different green descriptor (from the global hex array) on each call. drawTable(blue)
// draw table for one of six color cube panels function drawTable(blue) { // open table (one of six cube panels) document.write('<TABLE CELLPADDING=0 CELLSPACING=0 BORDER=0>') // loop through all non-dithered color descriptors as red hex for (var i = 0; i < 6; ++i) { drawRow(hex[i], blue) } // close current table document.write('</TABLE>') }

This function is very similar to the drawRow() function. The only difference is that it draws a 6 x 6 table, instead of a 6 x 1 row. It calls the drawRow() function six times, each time with a different red descriptor. drawCube()
// draw all cube panels inside table cells function drawCube() { // open table document.write('<TABLE CELLPADDING=5 CELLSPACING=0 BORDER=1><TR>') // loop through all non-dithered color descripters as blue hex for (var i = 0; i < 6; ++i) { // open table cell with white background color document.write('<TD BGCOLOR="#FFFFFF">') // call function to create cube panel with hex[i] blue hex drawTable(hex[i]) // close current table cell document.write('</TD>') }

This function accepts a single argument representing a decimal number, normally between 0 and 255. It converts it to its hexadecimal equivalent. At first, a list of the hexadecimal digits is assigned to the string variable hexCharacters. It returns the minimum 00 string if the decimal argument is less then 0, and the maximum FF if the decimal argument is greater than 255, the equivalent to FF in hex representation. The value Math.floor(dec / 16) is equal to the decimal representation of the first hexadecimal digit in the converted number. The value Math.floor(dec % 16) is equal to the decimal value of the second hex digit of the converted number. The hexadecimal value of a decimal number from 0 to 15 is that hexCharacter’s character, the index of which is equal to the decimal number.

This function is responsible for the fade effect. It accepts seven arguments; the first three represent the RGB descriptors of the initial background color, the next three represent the RGB values of the target one, and the last argument determines the speed of the fade. The function consists of a single loop which iterates from 0 to step in increments of one. The RGB descriptors of the current background are computed and assigned to local variables on each iteration of the loop. A computed red, green, or blue color is a weighted average between the initial color value and the target one. During the first iteration, step – i is equal to step, so the initial color descriptor is actually multiplied by 1 in the expression Math.floor(scol * ((step – i) / step). Since i is 0, the descriptor of the target color is multiplied by 0 in the expression ecol * (i / step) where col is red, green, or blue. Therefore, the background color is set to the initial color during the first pass and to the target color during the last pass. The last statement in the loop actually sets the background color by passing the computed descriptors to setbgColor(). Event Handlers It is common to use the fading effect with the onLoad and onUnload event handlers. If you prefer to use it with both, it is recommended that you generate the reversed transition for each.

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fgColor
The document.fgColor property represents the color of the document text (foreground color). It reflects the <BODY> tag’s TEXT attribute. The property is expressed as a hexadecimal RGB triplet or as one of the supported color names. When you assign it a triplet, the crosshatch mark (#) is optional. Setting a value to the fgColor property is equivalent to setting a value to the <BODY> tag’s TEXT attribute, enclosing the entire text with the <FONT COLOR="setColor"> </FONT> tag pair, or using the String object’s fontcolor method.

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alinkColor
The document.alinkColor property is JavaScript’s reflection of the <BODY> tag’s ALINK attribute. It is a string specifying the color of an active link (after the mouse button is pressed down over a link but before it goes back up). The string must represent the hexadecimal RGB triplet of a color or one of the supported color names. Aside from a few exceptions, you cannot set the value of this property after the page has finished loading, because there is no way to modify a page’s content after it has been laid out.

linkColor
The document.linkColor property is JavaScript’s reflection of the <BODY> tag’s LINK attribute. It specifies the color of the document’s hypertext links which the user has not visited yet. The color must be represented in the form of a hexadecimal RGB triplet or one of the supported color names. As explained above, you cannot set this property after the HTML source has gone through the layout stage. You can still read the property’s value at any time via an immediate script or a deferred one.

vlinkColor
The document.vlinkColor property is JavaScript’s reflection of the <BODY> tag’s VLINK attribute. The color must be represented in the form of a hexadecimal RGB triplet or one of the supported color names. This property represents the color of already-followed hypertext links. You can set it as long as the HTML source has not been through layout yet.

Output Methods and Streams
write and writeln
The document.write method displays any number of expressions in a document window. Expressions to be printed can be of any type, including numerics, strings, and logicals. This method prints its arguments to the plain HTML document window. It does not append any external character to the printed arguments. The method

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B Fontdocument.write, also accessible as window.document.write, can be used from either a plain script (<SCRIPT LANGUAGE="JavaScript">…</SCRIPT>) or from an event handler. Bear in mind that event handler scripts are executed only after the HTML source has been through layout. The write() method implicitly opens a new document of mimeType text/html if you do not explicitly invoke a document.open() method prior to the document.write() call. The writeln() method acts exactly like the write() method, except that it appends a new line character to the end of the output. HTML generally ignores this character, but certain tags, such as <PRE>, use it:
<PRE> one two three </PRE>

After interpretation, the web page appears as:
one two three

You can create the same output via JavaScript in the following fashion:
document.write("<PRE>") document.writeln("one") document.writeln("two") document.writeln("three") document.write("</PRE>")

Data Streams
The document.open() method opens a stream to collect the output of write() and writeln() methods. Its general syntax is:
document.open(["mimeType"])

mimeType specifies the type of document, which is one of the following:
text/html text/plain image/gif image/jpeg image/x-bitmap plugIn

plugIn is any two-part plug-in supported by the user’s browser. Generally speaking, if mimeType is text or image, the stream is opened to layout, which is generated by instructions from the browser. Otherwise, the stream is opened to a target plug-in which you have to make sure understands the data you provide.

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Since document is a property of window, document.open() or window.document.open() opens a stream specific to the document in the target window. If a document already exists in the target window, the open method clears it. If you do not specify mimeType as the method’s argument, the most common one, text/html, is assumed. Note that you should never use this method to open a stream in the document that includes the JavaScript method itself. It is always used to open data streams to documents in other windows. After you complete supplying data to the opened stream, you should close it via the document.close() method. Its syntax is simply the following:
document.close()

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This method primarily closes a stream opened with the document.open() method. If not explicitly closed by the script, all font style tag pairs are closed implicitly. For example, if you provide a <BIG> tag but do not provide a closing </BIG> tag later, JavaScript provides it automatically. The close() method also stops the “meteor shower” in the Netscape icon or the rotation of the IE icon, and displays “Document: Done” in the status bar. The document.close() method is extremely important because it instructs the browser’s interpreter to display the data stream. If you do not invoke it, the output might not have any influence on the content of the page. Since we have not discussed windows yet, this discussion seems a bit theoretical. We will refer back to these methods later, when the subject of windows will be dealt with in depth. Another related method is document.clear(). Clearing a document via this method clears all HTML outputs in that document and resets the object model corresponding to that document. Normally, since JavaScript automatically clears the old document when you open a new stream, you don’t have to clear a document prior to its opening or rewriting. The only case in which you should clear a document is after you close the stream to it. Since the method document.clear() does not work in versions of Netscape prior to 4.5, you can clear a document by opening it, writing a line break to it, and then closing it. Look at the following example:
windowReference.document.open("text/html") windowReference.document.write("<BR>") windowReference.document.close()

For some reason, this clearing method requires writing at least one character to the stream. The line break is used here because it is transparent to the user.

Summary
In this chapter I showed you several properties and methods of the document object. The focus was on colors, and naturally on hexadecimal triplets that define them. You learned how to script the various document colors such as background color, link color,

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and so forth. Two interesting examples dealing with colors were also analyzed. We have discussed other properties and methods as well. Additionally, I presented the basic output methods, document.write() and document.writeln(), as well as data streams. Data streams and document clearing play an important role in scripting windows and frames, as will be explained later.

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Forms
What are HTML Forms?
HTML forms, consisting of buttons, menus, and text boxes, are the means by which the client computer can gather information from the user. Forms create a graphical interface whereby the user can enter data in a logical and easy-to-follow method. Forms are supported by all the major browsers so you can use them with confidence. Form tags have been part of HTML since HTML 2.0 (the current specification as of this writing is HTML 4.0) and are supported by all JavaScript-enabled browsers. This is one of the reasons why forms are heavily supported by JavaScript. As will be explained later, JavaScript provides a convenient means of form content manipulation and validation through the use of a client-side application.

JavaScript Form Reference
JavaScript enables you to interact with the user via forms. You must know how forms are referenced in order to implement them in scripts. A form in terms of JavaScript is a plain object. It has properties, methods, and even event handlers. There are quite a few possible references from which you may choose. In this section I will outline all of these possibilities so you will have the freedom to select the most convenient method.

forms Array
Suppose you have an HTML document which includes several forms, each defined by a regular <FORM></FORM> tag pair. You can refer to each form by its index in the forms array. The forms array is a property of the document object so it is referred to as document.forms. The object representing the first form in the page is document.forms[0], the second form is document.forms[1], and so forth. The forms array includes an entry for each of the document’s forms (<FORM> tag), in source order. The general reference to a form is as follows:
document.forms[index]

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As with all of JavaScript’s arrays, the forms array includes a length property representing the number of forms on the page. The last form in the document, therefore, is:
document.forms[document.forms.length – 1]

Elements in the forms array can be set only by the HTML document and, hence, they are read-only for JavaScript. The following statement, for example, has no effect:
document.forms[0] = "work hard"

The string value of a form is <object nameAttribute>, where nameAttribute is the NAME attribute of the form.

Form Name
You can refer to a form by its name, rather than by its index:
document.formName

In order to take advantage of this referencing method, you have to explicitly assign a name to the form, via the NAME attribute.

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Note: The term reference is used to describe an object’s scripting protocol. A single object can be referenced via different but equivalent protocols.

form Object
HTML Syntax
All forms are basically just another set of HTML tags. The top-level tags are the <FORM></FORM> tag pair. All form elements must be placed within these tags, or else they are not interpreted correctly. The general syntax of the <FORM> tag is:
<FORM [NAME="formName"] [TARGET="windowName"] [ACTION="serverURL"] [METHOD="get" | "post"] [ENCTYPE="encodingType"] [onSubmit="handlerText"] [onReset="handlerText"]> </FORM>

The attributes are: n NAME—specifies the name of the form. This attribute is seldom used because it does not have any effect when using a server-side script. When using client-side JavaScript, though, it is recommended you name the form for easier referencing. Since a form’s name will be mostly used by JavaScript’s scripts, it is also preferred to use the JavaScript’s identifier naming standards.

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Although the <FORM></FORM> tag pair represents an HTML form, you can still place any other valid HTML tags within it. Mixing tables with forms, for example, is often used to enable simple layout. Although syntactically valid, nesting a form inside another form does not make any sense and you should avoid doing it.

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Note: Microsoft Internet Explorer allows you to place form elements outside the <FORM></FORM> tag pair. You should avoid using such elements because they are not accessible via a full object hierarchy. They are accessible, though, via JavaScript’s this scheme (see the explanation on this later in this chapter).

Event Handlers
onSubmit A submit event occurs when a form is submitted, an event reflected by the onSubmit event handler. This attribute is a must, or else there won’t be any response to the form’s submission. The onSubmit event handler is an attribute of the <FORM> tag because its action relates to the entire form, not only to its Submit button. A form can be submitted in several ways; the Submit button is only one of many. The submit event occurs immediately upon clicking the Submit button, pressing Enter, or via any other method. Since JavaScript triggers the event prior to sending the data to the server, the event handler’s script is executed before the form’s data is actually submitted to the server for further processing. Timing is very important here.

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TARGET—specifies the window to which form responses go. This attribute instructs the browser to display the server responses in the specified window rather than in the default window where the form resides. The specified value cannot be a JavaScript reference to a window—it must be a plain HTML frame or a window reference. ACTION—specifies the URL of the server-side script that processes the data submitted by the form. This attribute is necessary only when the processing script resides on the server. In this case, the script will be written in either C or Perl and will adhere to the CGI (Common Gateway Interface) protocol. The URL scheme must be HTTP. METHOD—specifies how to submit the form. It can be either get or post. The latter is more popular because it enables the client to send a greater amount of data to the processing script. Nonetheless, get is much easier to use and is also suitable for JavaScript scripts. If a form is returned with get, the data is placed in the QUERY_STRING environment variable. post, on the other hand, instructs the client to pass the data to the server via its operating system’s standard input method. ENCTYPE—specifies the MIME type of the submitted data such as “text/plain” for plain text. The default MIME encoding of the data sent is “application/xwww-form-urlencoded”.

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For example, suppose you ask the user to send you comments by filling in a text box in a form you place on your page. You might want to thank the user by replacing his comments in the text area box with a “Thank You” notice. However, you cannot use the onSubmit event handler to do that because the “Thank You” notice will replace the user’s input before it would have a chance to be submitted to the server. The net effect would be that you will receive the “Thank You” notice instead of the user’s comments. One way to work around this problem is to use an alert box instead of overwriting the form’s text area box. The onSubmit event handler is commonly used to validate the content of a form’s element. Client-side form validation is gaining popularity because, rather than waiting for a server-side CGI script to respond, the user receives an immediate response regarding invalid entries. Let’s say you have a form with a text box in which the user is asked to type his or her e-mail address. You can use a simple JavaScript script that will make sure (upon submission) that the user’s entry is a string containing an “at” sign (@), which is necessary for all e-mail addresses. You can use the onSubmit event handler not only to validate the form’s elements but also to cancel its submission altogether. The form’s submission is aborted when the event handler returns a false value, as in the following example:
<FORM NAME="form1" onSubmit="return false">

Obviously, this example is not very useful because it disables the form submission unconditionally. Usually, a function validates the form and returns a true or false value accordingly. You can use the following structure to cancel or proceed with the form submission, according to the value returned by the function:
<FORM NAME="form1" onSubmit="return checkData()">

The following example shows how to create a form with a text box and a Submit button which e-mails you the contents of the text box after prompting the user for confirmation:
<SCRIPT LANGUAGE="JavaScript"> <!-function proceedSubmission() { return confirm("Click OK to mail this information") } // --> </SCRIPT> <FORM ACTION="mailto:chuckeasttom@yahoo.com" METHOD="post" ENCTYPE= "text/plain" onSubmit="return proceedSubmission()"> <TEXTAREA NAME="inputField" COLS=40 ROWS=10></TEXTAREA><BR> <INPUT TYPE="submit" VALUE="mail it!"> </FORM>

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Figure 20-1. Using onSubmit in a form.

The Boolean value returned by the onSubmit event handler is actually the result of a confirm box presented to the user. Although some elements of this form are discussed later, you should be aware that, in order to receive the form’s content as a plain, unscrambled e-mail, you need to assign a "text/plain" value to the ENCTYPE attribute. onReset Another event handler of the <FORM> tag is onReset. A reset event usually occurs when the user clicks the Reset button. Except for the triggering event, the onReset event handler behaves like the onSubmit event handler. The following example asks the user to confirm the resetting process before executing:
<FORM ACTION="mailto:chuckeasttom@yahoo.com" METHOD="post" ENCTYPE="text/plain" onReset="return confirm('Click OK to reset form to default status')"> <TEXTAREA NAME="input" COLS=40 ROWS=10></TEXTAREA><BR> <INPUT TYPE="reset" VALUE="reset it!"> </FORM>

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Note:

The onReset event handler was first implemented in Navigator 3.0x.

submit() The submit() method submits a form much the same way as the Submit button. The submit() method sends data back to the HTTP server either via get or post submission schemes. The general syntax is as follows:

Example 20-1 (ex20-1.htm). A hypertext link used to submit a form just like a Submit button.

In Example 20-1 the form is submitted by the submitForm function which is invoked via the onClick event handler of a link object. After prompting the user for confirmation via the onSubmit event handler, the form, referenced as document.forms[0] (it is the first and only form in the page), is submitted through its submit() method. Example 20-1a shows the file thanks.htm which displays the “Thank You” notice referenced in Example 20-1:
<HTML> <HEAD> <TITLE>Thank you</TITLE> </HEAD> <BODY> Thank you very much for your feedback </BODY> </HTML>

Note: The submit method is broken in versions of Navigator prior to 4.5 and versions of Internet Explorer prior to 4.0. Test every new release before usage.

reset() The reset() method resets a given form and is equivalent to clicking the Reset button. Its syntax and usage is the same as the submit() method. See the onReset event handler for more information on resetting a form.

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Properties
action The action property reflects the value of the <FORM> tag’s ACTION attribute. Its value is the URL of a CGI or LiveWire application on the server which needs to execute upon submission. If no explicit URL for the server-side application is specified, the value of action in Navigator defaults to the URL of current document (the one containing the form), while Microsoft Internet Explorer defaults the property to an empty string. The formReference.action property can also be assigned a value by JavaScript. You can take advantage of this feature to modify the form’s behavior after the page has been laid out, according to the user’s preferences. In general, you can assign the form’s properties by JavaScript, instead of HTML attributes. Take a look at the following form example:
<FORM NAME="form1" METHOD="post" ACTION="http://www.foo.com/trash.cgi"> </FORM>

An alternative to this construct is the following combination of HTML and JavaScript:
<FORM NAME="form1" METHOD="post"> </FORM> <SCRIPT LANGUAGE="JavaScript"> <!-document.form1.action = "http://www.foo.com/trash.cgi" // --> </SCRIPT>

elements The elements property is an array of objects corresponding to the form elements. It is a property of any form and is referenced as formReference.elements. As you already know, since all form objects are elements of the document.forms object, it is possible to refer to a specific form within a document if you know its index in relationship to the other forms in the document. Similarly, a specific form’s elements are elements of the elements array. The first element of this array is a reference to the first form element (button, checkbox, hidden, password, radio, reset, select, submit, text, or textarea object), the second entry is a reference to

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the second form element, and so on, in source order. For example, if a form has one text box and three radio buttons, you can reference these elements as formReference.elements[0], formReference.elements[1], formReference.elements[2], and formReference.elements[3]. Like the forms array, the elements array is an alternative to referencing by name. Some programmers prefer to use array reflection, whereas others prefer to trace elements by their names. It is convenient to implement the elements array when a form contains many elements that are related to indexes. A form with ten text boxes that each accept a telephone number, for example, should be referenced via the elements array. Such referencing will allow you to use a loop to iterate through the ten different elements. Usually, though, you will use forms to collect data fields of different meanings which are not similar to each other as the phone numbers in the previous example. In such cases, referencing by name is much more convenient and easier to understand, follow, and maintain. You can easily redesign the physical layout of the form or even add new elements in the middle while keeping the old references. If you use the elements array, on the other hand, any layout modification causes the indexes to shift, invalidating all previous references. The number of elements in a form is formReference.elements.length. Therefore, the last element in a form is reflected by the formReference.elements[formReference.elements.length – 1] entry. Elements in the elements array are read-only, so the following statement, for example, has no effect:
formReference.elements[0] = "do not do this"

The first few elements of the elements array represent the form elements. The following property, length, reflects the number of form elements. You would expect the list to end here but Netscape added all forms properties to the elements array, starting at this index. Suppose a form element evaluates to foo. Then, the following references all evaluate to foo:
document.forms[0].elements[0].value document.forms[0].elements.elements[0].value document.forms[0].elements.elements.elements[0].value

These bizarre references are possible due to the fact that an elements object contains all the properties of its form, in addition to the form’s elements. A form, say document.forms[0], can also have a property named name reflecting the value of the NAME attribute of the corresponding <FORM> tag. You can also reference this property in Navigator using one of the following known methods:
document.forms[0].name document.forms[0].elements.name document.forms[0].elements.elements.name

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You may recall from Chapter 4 that the for…in loop statement provides the capability to list an object’s properties, top to bottom. The following function can be used to list the properties of a typical elements object by name (not index):
function printElements(form, formString) { // initialize output string var result = "" for (var i in form.elements) { result += formString + ".elements." + i + " = " + form.elements[i] + "\r" } alert(result) }

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You can use this function by invoking it with a specific form’s reference and a name. Consider the following simple HTML form:
<FORM NAME="form1" ACTION="http://www.yourserver.com/filename.cgi" METHOD="post"> <INPUT TYPE="text" NAME="num1" VALUE="bla"> <INPUT TYPE="hidden" NAME="num2" VALUE="foo"> <TEXTAREA NAME="num3">wow</TEXTAREA> </FORM>

The preceding printElements() function returns the following output when provided with the arguments document.forms[0] and "document.forms[0]":

Figure 20-2. A list of elements object’s properties.

Notice that the first four lines list the exclusive properties of the elements object. The following lines represent properties of elements that are identical to those of the form itself, document.forms[0] in this case. Theoretically, you can reference all forms properties of a given form as properties of its elements object. If you think this wealth of referencing methods is confusing, you are right. The best solution is to stick to some standards. Suppose you have a form in a document that is referenced as document.forms[index], document.formName, or formReference. The following objects include all forms properties:

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formReference formReference.elements

The elements property of formReference is just another interface to the same object. Since Netscape originally documented the above two objects as different ones (and still does), it is common to refer to formReference when accessing a form’s general properties and to formReference.elements when accessing the properties of the form’s elements. Theoretically, you can also access the first element of the form by:
formReference[0]

As you already know, any form element object can be referenced by its name. For example, a text object whose name is field1 in the first form of a page can be accessed as follows:
document.forms[0].field1

When two or more elements have the same name, they form an array in which the indices are determined according to their layout order. For example, if there are three text objects in one form, all named inputField, you can reference these elements in the following fashion:
document.forms[0].inputField[0] document.forms[0].inputField[1] document.forms[0].inputField[2]

encoding The content of a form is encoded before it is submitted to the server. There are various types of encoding, or MIME encoding—some suitable for files, while others are suitable for plain text or other purposes. The encoding method is initially specified by assigning it to the <FORM> tag’s ENCTYPE attribute. The default encoding method is application/x-www-form-urlencoded, but others such as multipart/form-data and text/plain are also available. Every HTML form has a MIME encoding specification, even if it is not explicitly shown. The value of JavaScript’s formReference.encoding is initially the value assigned to the HTML ENCTYPE attribute. This property is not read-only, so it can be set at any time, even after layout has been completed. Setting encoding overrides the ENCTYPE attribute and instructs the browser (primarily Navigator) to use the new MIME encoding method. For your reference, the general syntax of this property is:
formReference.encoding

If no value is specified as the ENCTYPE attribute of a form, it is defaulted to application/x-www-form-urlencoded. Nevertheless, the value of formReference.encoding remains an empty string. method The METHOD attribute of a <FORM> tag accepts either a get or a post value. JavaScript reflects the value of this attribute in the form of a method property. You can use this

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property like all other form properties. It can be read or set at any time, even after layout has been completed. The term “method,” in this case, has no relation whatsoever to the object-oriented interpretation of the word (a function associated with an object). The formReference.method property is active when the client-side JavaScript script interacts with a specific server-side CGI or LiveWire application. In this case, you can set the method property along with other form-related properties to modify the form’s layout on the fly. The general syntax to reference this property is:
formReference.method

target When you “surf the web,” you often encounter HTML forms that submit data to the server and return a result, such as a list of sites or a simple “Thank You” page. Since most forms serve as a means to interact with the user, you will seldom find forms that submit to the server and do not respond. Furthermore, even forms that just receive input from the user and submit it to a server-side script are expected to respond and give the user an indication that the content of the form was correctly submitted. Most server-side applications return a new HTML page in the same window as the form. Sometimes, however, you intend to receive the results in a different window or frame. You can specify the target of the returned page by setting the TARGET attribute of the <FORM> tag. This value is reflected by JavaScript’s target property, which belongs to the form object. The general syntax for referencing this property is:
formReference.target

This property can be both read and set at any time, even after the page has been laid out. Despite the fact that the TARGET attribute defaults to the current window or frame’s HTML document, the JavaScript’s target property does not default to any string. The value of JavaScript’s target property, like HTML TARGET attribute, can be either ’s a window or a frame name. In addition to these obvious values, there are several common references: _top, _parent, _self, and _blank. These values will be covered again when we discuss frames and windows and, for detailed information, you may also refer to HTML documentation. It may seem convenient to assign window or frame objects to this property, but unfortunately it only accepts HTML specifications.

Element Types
Each and every form element (button, radio, text area, etc.) features a type property that reflects the type of that form element. As usual in JavaScript syntax, the type property follows the element reference. For example, you can access the type of the first element of the first form in the following fashion:
document.forms[0].elements[0].type

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The default value of the HTML METHOD attribute is get. The value of the JavaScript method property, however, has no default and stays empty.

All values listed in the right column are plain strings representing the element type.

Using this with Event Handlers
When you call a function via an event handler you may refer to the form element that triggered the event handler, such as a text object or a button. The following script segment and figure demonstrate this concept:
<SCRIPT LANGUAGE="JavaScript"> <!-function process() { document.forms[0].elements[0].value = "thank you" } // --> </SCRIPT> <FORM> <INPUT TYPE="text" NAME="myField" VALUE="email..." onChange="process()"> </FORM>

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Figure 20-3. Using the onChange event handler.

At this point, it is not so important to understand what exactly this script does. The INPUT TYPE="text" definition creates a simple text box in which the user can enter a value. The text box (or text object) is assigned the string "email..." as its default value (the form comes up with this text inside the box). The onChange event handler captures the change event that occurs when the user changes the value of the text object and clicks outside of it. When such an event occurs, the function process is invoked and assigns the string "thank you" to that text object’s value property. Notice that a full object path specification, from the document browser object downwards, is used to access the text object. Such referencing has two disadvantages: n The path is fairly long and complex. n If you change the position or the name of either a form or an element, the path must be modified to reflect this change, making maintenance very difficult. The answer to this problem is using the keyword this to refer to the “current” object. For example, you can simplify the preceding code by implementing the this reference in the following way:
<SCRIPT LANGUAGE="JavaScript"> <!-function process(callingElement) { callingElement.value = "thank you" } // --> </SCRIPT> <FORM> <INPUT TYPE="text" NAME="myField" VALUE="email..." onChange="process(this)"> </FORM>

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Using the above scheme, you can change the element’s position, name, or any other optional attribute (other than the event handler) and the script will still work without any modifications. The keyword this refers to the element providing the event handler. In this case, the value of this is equal to document.forms[0].elements[0]. When the function process is called, the value assigned to the callingElement parameter is this, so callingElement.value is equivalent to document.forms[0].elements[0].value. The keyword this within an event handler script refers to the form element to which the event handler belongs. For example, the keyword this in an event handler that belongs to the first element of the first form in a document can be safely replaced by document.form[0].element[0]. Using the object this is very convenient when you use a single function to refer to different form elements. You can also use this in an event handler script for purposes other than a function’s argument. You can also hand any property of the this object to a function, as demonstrated by the following script segment:
<SCRIPT LANGUAGE="JavaScript"> <!-function display(str) { alert(str) } // --> </SCRIPT> <FORM> <INPUT TYPE="text" NAME="myField1" VALUE="d" onChange="display(this.value)"> <INPUT TYPE="text" NAME="myField2" VALUE="f" onChange="display(this.value)"> </FORM>

þ

Note: Object, property, and method references can be passed as function arguments. Just like assigning any other value, they can be assigned to a variable as well. For example, you can use the following script segment instead of a window.document.write statement:
var obj = window.document obj.write("Cool<BR>")

Be careful not to enclose an object reference in quotation marks—it is not a string.

Until now, the special object this was used as a substitute for a form element’s full path. JavaScript also allows you to reference a form from an element’s event handler script via the form property of this object. The previous script segment can be rewritten in the following format:
<SCRIPT LANGUAGE="JavaScript"> <!-function process(callingElement)

The object representing the form is equivalent to document.forms[0]. Generally speaking, every form element includes a form property that enables a reverse access; that is, ability to reach the form from its element (although the element is really the form’s property). Therefore, you can use any one of the following expressions to access the first element of the first form in a given document:
document.forms[0].elements[0] document.forms[0].elements[0].form.elements[0] document.forms[0].elements[0].form.elements[0].form.elements[0]

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You will probably never use this property independently in a script, because you can always refer to a form directly as a property of a window’s document object. However, such a reference is used a lot with forms, because an event handler’s script references the event handler as this, and the form property enables you to reference the form through a back door.

þ

Note: Microsoft Internet Explorer versions 3.0 to 5.5 do not properly support the use of this in all form element event handlers. Double-check every script that uses this object.

In addition to event handlers associated with form elements, you can also use this with event handlers of the <FORM> tag. In this case, this represents the object encompassing the entire form (such as document.forms[0]). Suppose you want to call a function from an onSubmit event handler. You can use the following outline to hand an object reference representing the form object to the function:
<FORM ... onSubmit="functionName(this)">

In this case, the expression this.form has no logical meaning.

Utilizing the form Property
In this chapter we discuss the various objects reflecting HTML form elements. Each form element is a direct property of the form to which it belongs. Take a look at the following example:
<FORM NAME="myForm"> <INPUT TYPE="text" NAME="myField" SIZE=10> </FORM>

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In this simple example, the text object may be referenced as document.myForm.myField. This top-to-bottom hierarchy enables you to access any form element object if you already have an access to the form object itself. However, you may encounter a situation in which you pass a form element object to a function, for instance, and you want to reference the form object through a back door. JavaScript enables you to do so with the form property. For example, suppose you have a variable myField which holds a form element object. (For now, simply ignore how the object was assigned to the variable.) Assume the variable is named objRef and the object reference was explicitly assigned to it by the following statement:
var objRef = document.myForm.myField

Bear in mind that you do not have this statement in the script—all you have is the variable objRef, and you are attempting to reference the form object, myForm. You can use the form property to do so:
var formObjRef = objRef.form

form is a property of every form element object, with no exceptions. It is very convenient to invoke a function from an event handler with this as an argument. For example, consider the following script segment:
<SCRIPT LANGUAGE="JavaScript"> <!-function getValue(otherElement) { alert(otherElement.form.elements[1].value) } // --> </SCRIPT> <FORM> <INPUT TYPE="button" VALUE="click me" onClick="getValue(this)"> <INPUT TYPE="text" VALUE="Baruch Spinoza" SIZE=10> </FORM>

Figure 20-4. Using the form property.

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The first text object (form element) invokes the getValue function with this object as the argument. The function is designed to print the value of the second element, elements[1]. The only relation between the value handed to the function (the object representing the first form element) and the second form element is that they are both “children” of the same “parent.” The function accepts the object reflecting the first element, so it must access the second element through the common parent, the form object. Therefore, the correct syntax must include the form property (which is an object as well):
firstElement.form.secondElement

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The form property acts like a connector in this case. If you implement a function that references various elements of a form, you may want to initially use this.form as the function’s argument. You should then use the following code in place of the preceding script segment:
<SCRIPT LANGUAGE="JavaScript"> <!-function getValue(form) { alert(form.elements[1].value) } // --> </SCRIPT> <FORM> <INPUT TYPE="button" VALUE="click me" onClick="getValue(this.form)"> <INPUT TYPE="text" VALUE="Plato" SIZE=10> </FORM>

The NAME attribute enables you to assign this HTML object a name which identifies it in both server-side queries and JavaScript scripts. The VALUE attribute accepts the initial string that should appear in the box when the page loads. Not only is this string the initial one in the text box, but it is also the default string. When you reset the form via a Reset button or the reset() method, this string reappears in the text box. The

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SIZE attribute is essential because it determines the size (in characters) of the text box. The MAXLENGTH attribute specifies the maximum input length allowed in this field. If the user enters a string that is longer than MAXLENGTH, only the first MAXLENGTH characters are entered. This option is especially handy when requesting a specific string, such as a password, which is naturally limited to a given length. The following script segment demonstrates the use of the text object in an HTML document:
Processor: <INPUT TYPE="text" NAME="comp" SIZE=15 MAXLENGTH=20 VALUE="Pentium Pro">

A text object can hold a maximum of one line. You can use a textarea object to display multiple lines.

You already know that a form can be accessed through its name or via the forms object when the form’s index is known. Similarly, you can access a form’s element by its name or through the elements array if the element’s index is known. The preceding expressions show how to access a text object by itself. Usually, you will not access the text object but rather its properties, methods, or event handlers.

Event Handlers
The text object is a very convenient means for both input and output. A text object has a wealth of event handlers you can use. onBlur A blur event occurs when a text field loses focus. A field gains focus when the user clicks inside the text box, and the focus is lost when the user clicks outside the box, anywhere on the page. The onBlur event handler executes JavaScript code when a blur event occurs. Take a look at the following form and function:
<SCRIPT LANGUAGE="JavaScript"> <!-function checkInput(element) { if (element.value == "") alert("Please enter a value!") } // --> </SCRIPT> <FORM> <INPUT TYPE="text" NAME="myField" VALUE="" onBlur="checkInput(this)"> </FORM>

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Figure 20-5. Using the onBlur event handler.

If the text box is left empty, an alert dialog box informs the user that he or she must enter a value. The dialog box is displayed only if the user first accesses the field and then exits it without entering any value. onChange A change event occurs when a blur event occurs and the value of the text object has been modified. The onChange event handler, also defined as an HTML tag attribute, executes JavaScript code when a change event occurs. The onChange event handler is probably the most commonly used among the event handlers related to the text object (box or field). You can use this event handler when validating data entered by the user. That is, when the user modifies the text box content, a function is invoked to validate the changes. Validation via JavaScript instead of CGI or LiveWire saves precious network transmission time. In order to demonstrate the onBlur event handler, a simple form element and a corresponding function have been implemented. The problem with this example is that the value of the text box is validated whenever it loses focus. However, it is generally better to validate the form only after changes are made. Therefore, the onChange event handler is superior for such tasks. The previous example is much improved in the following version:
<SCRIPT LANGUAGE="JavaScript"> <!-function checkInput(element) { if (element.value == "") alert("Please enter a value!") } // --> </SCRIPT>

The following rules are very important in order to fully understand this event handler: n A blur event naturally takes place whenever a change event occurs. n A change event takes place only when a blur event occurs and the value of the text object has been modified and gained focus. onFocus A focus event occurs when a field receives input focus by tabbing on the keyboard or clicking with the mouse. The onFocus event handler obviously executes a prespecified code when a focus event occurs. In versions of Internet Explorer prior to 5.0, a focus event occured when the page loaded. The following script scrolls a T-banner in a text object. It starts when the user clicks somewhere inside the text box (text object), triggering the onFocus event handler. Here is the script:
<HTML> <HEAD> <TITLE>T-Banner</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-// set speed of banner (pause in milliseconds between characters) var speed = 100 // decrease value to increase speed (must be positive) // set pause between completion of message and beginning of following one var pause = 1000 // increase value to increase pause // set initial values var timerID = null var bannerRunning = false // create global array var ar = new Array() // assign the strings to the array's elements ar[0] = "Welcome to our JavaScript page" ar[1] = "We hope you enjoy the T-Banner script" ar[2] = "It is designed to be more stable than regular banners" ar[3] = "Don't forget to check out our other scripts" // set index of first message to be displayed first var currentMessage = 0 // set index of last character to be displayed first var offset = 0 // stop the banner if it is currently running function stopBanner() { // if banner is currently running if (bannerRunning) // stop the banner clearTimeout(timerID) // timer is now stopped

Example 20-2. A T-banner starts in a text box when the user clicks in the box.

You can find a detailed explanation of this script in Chapter 16. Only new features will be covered here. First of all, notice the form named bannerForm in the HTML body. The form consists of only one element, a text object. The name of this object is bannerField. Its size is set to 50 characters, and its default value is the string “Click here...”. The onFocus event handler is provided. When the user clicks inside the text box, a focus event occurs and the startBanner() function is invoked to start the scrolling. Notice that this statement is executed only if the banner is not running. The variable bannerRunning holds the banner’s current state and is already true when the page loads. The only difference between the original status bar-based T-banner script earlier in this book and the text box-based one in Example 20-2 is that the property window.status is replaced by document.bannerForm.bannerField.value, which is the reference to the text object’s content-reflecting property. onSelect A select event occurs when the user selects part of the text within a text field. The onSelect event handler enables you to respond to such an event. Here is the general syntax for implementing this event handler:
<INPUT TYPE="text" VALUE="" NAME="valueField" onSelect="selectState()">

Since the select event is rather rare and insignificant, this event handler is not commonly used. It is also broken in versions of Navigator prior to 4.5.

Methods
blur() You already know what a blur event is. You can explicitly blur a text object using the object’s blur() method, which removes focus from the field. This method deselects any text that might be selected in the field and removes the text insertion pointer from the field. At this point, no fields or form elements are focused. A manual way to blur a text object is to hit Tab, which advances focus to the next field in order and removes it from the current field (blurring it). However, the JavaScript blur() method only removes focus from its object, without giving focus to any other field in the form or in the page.

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A read-only text field is a classic example for using the blur() method. The algorithm to create such a field is very simple. When the user explicitly gives focus to a field in order to write in it, an event handler (onFocus) invokes the blur() method to instantly remove the focus. Here is a read-only text field example:
<INPUT TYPE="text" NAME="myField" VALUE="" SIZE=15 onFocus="this.blur()">

You can consider the expression onFocus="this.blur()" an attribute of a read-only <INPUT TYPE="text"> element. focus() The focus() method focuses on a text object (or another form element object described later in this chapter). Focusing on a form element usually means that the window scrolls until the text field is viewable and the cursor is positioned at the beginning of the text in the field. Although not mandatory, it is good practice to give focus to a text object before accessing it. This will have a positive contribution to the script’s robustness. When the page contains many fields, you can use the script-driven focus() method to emphasize one of them, thus “attracting” the user to it. The focus() method resembles the camera’s focus: It emphasizes a specific object and prepares it for further use. The following HTML tag can be used to maintain focus on a text object (it may be useful if you have multiple text objects and the user is allowed to modify only one of them):
<INPUT TYPE="text" NAME="myField" VALUE="" SIZE=15 onBlur="this.focus()">

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When the text field loses focus, a blur event occurs, the onBlur event handler is triggered, and focus is given once again to the field. The focus() method usually belongs to text objects (fields), but, as will be explained later in this chapter, other kinds of objects use it as well. select() The most useful method of the text object is the select() method. When you select a field via JavaScript, the entire text in the field is selected. This method is broken in Microsoft Internet Explorer 3.0 and previous versions. In Navigator on some platforms, you must give focus to a text field before selecting it. Suppose you have a document with one form that contains one element, a text object. The following script segment is needed in order to select that text field via JavaScript:
document.forms[0].elements[0].focus() document.forms[0].elements[0].blur()

The select() method is extremely useful and very convenient when validating a form on the client side. For example, let’s say you have a form that accepts inputs from the user for several elements and validates each element in turn before submitting the data to the server. The most basic way to report an error is to display a message in an alert dialog box. More sophisticated error reporting includes automatic preparation of

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the form for the user’s corrections. Such preparation can be implemented by using the focus() and select() method together. When the script encounters a text field that contains invalid data, you can direct the cursor to that field and automatically highlight the interior text. The user can then write the new, correct value without having to delete the invalid entry. Sounds quite complicated on the programmer’s behalf, but it is actually very simple. The following script and form demonstrate a simple validation and handy error reporting:
<HTML> <HEAD> <TITLE>Simple form validation</TITLE> </HEAD> <SCRIPT LANGUAGE="JavaScript"> <!-function checkName(field) { if (field.value == "") { alert("Value is required") field.focus() field.select() } else if (field.value.split(" ").length < 2) { alert("Enter full name") field.focus() field.select() } } function checkEmail(field) { if (field.value.indexOf("@") == –1) { alert("Enter a valid e-mail address") field.focus() field.select() } } // --> </SCRIPT> </HEAD> <BODY> <FORM> Full name: <INPUT TYPE="text" NAME="userName" VALUE="" SIZE=15 onChange="checkName(this)"> <BR> Email address: <INPUT TYPE="text" NAME="email" VALUE="" SIZE=15 onChange= "checkEmail(this)"> </FORM> </BODY> </HTML>

Example 20-3. A simple form validation and error handling script.

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The form in Example 20-3 includes two text objects: the user’s full name and e-mail address. The element (text field) object is passed as an argument to the two different event handler functions. The value property reflects the current string in the text field. checkName() checks that the value of its calling object (text field) is not empty and that it contains exactly two words (with a separating space). If one of these rules is violated, an appropriate message is displayed, and the calling element (the userName text object) is focused and selected, indicating the invalid text field. checkEmail() checks if the value of its calling text object contains an “at” character (@). If one is not found, an appropriate message is displayed, and the calling element (the email text object) is focused and selected. The text within the field is highlighted by the select() method of the second text object in the form. As you can see, the script conveniently uses this for all references to the form element objects. Also notice that field checking is done per every mouse click anywhere inside the window.

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Properties
defaultValue The defaultValue property is a string indicating the default value of a text object. The default value is initially assigned to the VALUE attribute of the HTML tag, reflected also by JavaScript’s defaultValue property. Setting this property in JavaScript overrides the initial HTML values. You can set and read the defaultValue property at any time, even after layout has been completed. Since it reflects the field’s default value only (the one seen when the page loads or after resetting the form), this property cannot be used to dynamically update the text in the field. If the HTML attribute VALUE is not specified, defaultValue defaults to an empty string, as if explicitly specifying VALUE="". Microsoft Internet Explorer did not support this property until version 5.0. The following script segment demonstrates a simple use of the defaultValue property to reset only a specific form element (a text object in this case), rather than resetting the entire form with the Reset button or the reset() method. Note that this script uses the value property which we discuss later.
<SCRIPT LANGUAGE="JavaScript"> <!-function resetField(sub) { document.forms[0].elements[sub].value = document.forms[0].elements[sub].defaultValue } // --> </SCRIPT> <FORM NAME="fields"> <INPUT TYPE="text" NAME="field1" VALUE="enter first" SIZE=15> <A HREF="javascript:resetField(0)">reset</A><BR> <INPUT TYPE="text" NAME="field2" VALUE="enter second" SIZE=15> <A HREF="javascript:resetField(1)">reset</A><BR>

Since we haven’t discussed buttons yet, this script uses links for invoking JavaScript functions. A link is not a form element, so you cannot refer to this or this.form inside the resetField() function. To work around this problem, each link is identified by the index of the corresponding text object and is handed to the function as an argument. The first link, for example, is responsible for resetting the first text object in the form, so it hands a 0 to the resetField() function. Instead of resetting the form element, the function sets the current value of the corresponding text field to its default value.

name
It is generally good practice to name every form element, especially the text objects. Names are mandatory for CGI scripts, which use them for field identification and value extraction. Since it allows a more logical and convenient access to JavaScript text objects, identifying text objects by names contributes significantly to the script’s robustness and ease of maintenance. All names, therefore, should be meaningful and adhere to JavaScript’s naming conventions. The name property initially reflects the value of the HTML NAME attribute. Changing ’s the value of this property overrides the initial HTML setting. By assigning a new string to the name property, you can set the name of a text object at any time. Do not confuse this property with the text that appears in the field. A field’s content is represented by the VALUE attribute, rather than the NAME attribute. Hence, if you modify the value of name, you will notice no change in the page’s appearance. You should recall that a form element can be referenced by its index or by its name. If a form contains multiple elements with identical names, they form an array. Although this situation is especially common to radio buttons, as you will see later in this chapter, you may encounter it with text objects as well. The following example demonstrates the use of such arrays to handle complex forms:
<HTML> <HEAD> <TITLE>Form element arrays</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function correctName() { for (var i = 0; i < document.fields.username.length; ++i) { var field = document.fields.username[i] field.value = field.value.charAt(0).toUpperCase() + field .value.substring(1, field.value.length) } } function checkEmail()

Example 20-4. A form validation script based on arrays reflecting elements with identical names.

In order to understand this listing you must be aware of the page’s initial appearance.

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Figure 20-6. A form that collects data about several people.

As you can see, this form is divided into nine text objects. All six text objects for entering first and last names are named username. The value of document.forms[0].elements[0].name, for example, is equal to the string "username". The three text objects for entering e-mail addresses are named email. There are basically two element groups, username and email. Since all form elements are named fields, you can reference these two groups as arrays, via the following syntax:
document.fields.username document.fields.email

Both arrays have a length property, representing the number of elements sharing the given name. The length and elements of the username array, for example, can be referenced as follows:
document.fields.username.length // number of name-related elements document.fields.username[0] // first First Name object document.fields.username[1] // first Last Name object document.fields.username[2] // second First Name object document.fields.username[3] // second Last Name object document.fields.username[4] // third First Name object document.fields.username[5] // third Last Name object

The same syntax is also used to reference elements of the email array. Take another look at the script in Example 20-4. It includes two functions, one for handling name input fields and one for e-mails. The first function, correctName(), loops through all username elements and capitalizes their first letters. The second function uses the same technique to alert the user if any of the e-mail addresses do not include the “at” character (@). If such a field is found, the function focuses on and selects it, in order to attract the user’s attention to the invalid field.

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You should normally assign a distinct name to each and every form element. Example 20-4 demonstrates that in some cases, however, it is much more convenient to reuse a specific name in order to enable a loop-based processing of elements that share similar characteristics.

value
The value property reflects the current text located in a given text object (field). It is not equivalent to any HTML attribute. You can read and set this property at any time. The field’s content is updated immediately upon setting. Although we have been using this property throughout this chapter, here is another example which shows how to reference the value attribute of the first element of the first form in a page:
document.forms[0].elements[0].value

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password Object
HTML Syntax
Another possible value for the TYPE attribute is password. It is similar to the text object except that, instead of displaying the user’s input, it responds to any typed character with a predefined single character, such as an asterisk (*). The general syntax of this object is as follows:
<INPUT TYPE="password" NAME="passwordName" [VALUE="contents"] [SIZE="integer"] [MAXLENGTH="integer"] [onBlur="handlerStatement"] [onChange="handlerStatement"] [onFocus="handlerStatement"] [onSelect="handlerStatement"]>

The value property is an asterisk character (or any other character chosen by the browser). The additional properties are identical to those of the text input type. Here is a simple password box code:
Password: <INPUT TYPE="password" NAME="pswrd" SIZE=15 MAXLENGTH=20 VALUE="">

The value of the password box is the typed-in string, not the asterisks (or other characters) that appear. The value is hidden so it cannot be seen by someone near the user.

JavaScript Access
Like all other elements, there are four ways to access a password object via JavaScript:
[window.]document.formName.passwordName [window.]document.formName.elements[index]

All methods and element references adhere to one of these alternatives.

Event Handlers
The password event handlers are identical to those of the text object in all aspects. Refer to earlier discussion in this and other chapters.

Methods and Properties
The password object’s methods are identical to those of the text object: n focus n blur n select The password object’s properties are: n defaultValue n name n value The password object was not very useful in Netscape Navigator 2.0 because JavaScript was not allowed to access its value. Starting with versions 3.0, JavaScript on Navigator and IE can freely access the VALUE attribute as if it is a text object. In terms of JavaScript, the password object is identical to the text object. The only difference is that the value of the password field is never displayed.

textarea Object
HTML Syntax
Multiple lines of text can be entered into a form via text areas which are defined by the <TEXTAREA></TEXTAREA> tag pair. You should always name a text area and specify its size. The general syntax of the <TEXTAREA> tag is as follows:
<TEXTAREA NAME="textareaName" ROWS="integer" COLS="integer" WRAP="off | virtual | physical" [onBlur="handlerStatement"] [onChange="handlerStatement"] [onFocus="handlerStatement"] [onSelect="handlerStatement"]> textToDisplay </TEXTAREA>

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The attributes are: n NAME—specifies the name (label) of the text area element. It is very difficult to work with unnamed text areas, so always name your text areas according to JavaScript naming conventions. n ROWS—specifies the number of text rows in the text area. Since Netscape Navigator and Microsoft Internet Explorer use different size fonts in text fields, you cannot set the text area size in pixels by setting ROWS. n COLS—specifies the number of text columns in the text area. It is equal to the number of characters in one text line. It is possible to initialize the text area by placing text between the <TEXTAREA> and </TEXTAREA> tags in the following fashion:
<TEXTAREA NAME="comments" COLS=35 ROWS=5> Write any comments regarding this page here. Don't forget to mention your e-mail address. </TEXTAREA>

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Carriage return characters are interpreted as new lines in this text zone. Netscape Navigator enables you to write HTML tags in the text area by simply placing the script between the opening and closing <TEXTAREA> tags. Internet Explorer does not support this feature, though, so you should avoid using it altogether. Another attribute accepted by the <TEXTAREA> tag is WRAP. It can be set to one of the following values: n OFF—do not wrap lines (default value). n VIRTUAL—wrap lines on the screen but refer to them as one line otherwise. n PHYSICAL—insert actual line breaks (CR characters) in the string.

Event Handlers
The textarea object’s event handlers are identical to those of the text object.

Properties and Methods
Like event handlers, all properties and methods are exactly the same as those of the text object and password object. Please refer to previous listings for the text object in order to learn about its properties, methods, and event handlers.

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Inserting New Line Characters
A text area field is actually a multiline text field. Printing output to a textarea object is somewhat more complicated than printing to a text object due to the extra complication of new line insertion. This is especially important when the WRAP attribute is set to off and you are interested in avoiding a long horizontal text span. Since the new line character is not uniform across different platforms, inserting a new line is not as simple as inserting any other printable character. The new line character on Unix- and Macintosh-based machines is “\n”, while Windows operating systems require the “\r\n” pair. The simplest way to work around the problem is to test the actual platform and insert the corresponding new line character. The following is a function that automatically returns the correct string for the user’s platform:
function getNL() { if (navigator.appVersion.lastIndexOf('Win') != –1) return "\r\n" /* else */ return "\n" }

You can assign the new line character in the following fashion:
var NL = getNL()

Now, you can use the function’s return value to place a multiple-line text in a textarea object. Here is a simple example:
document.forms[0].elements[0].value = "line 1" + NL + "line 2"

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Important Note: Netscape Navigator supports “\n” as a new line character on all platforms. However, Microsoft Internet Explorer requires a platform-dependent new line character.

hidden Object
HTML Syntax
A hidden form field is one that is not displayed on an HTML page. It is used for passing unique identification values upon submission. Since the user cannot modify or interact with a hidden element, the initial value given when the page loads stays the same throughout the life of the page. The hidden object, then, is not useful for the user but rather for the programmer. On a large web site, a server-side application can use a hidden object to distinguish between different forms submitted to the server. Except that it is not viewable, you should basically treat the hidden element exactly like the text type. The element’s general syntax is:
<INPUT TYPE="hidden" NAME="hiddenName" [VALUE="textValue"]>

The NAME attribute specifies the name of the hidden object, whereas VALUE specifies the value of the field. Although not viewable, they are part of the form’s content, as any other element type’s attributes.

JavaScript Access
There are four ways to reference a hidden object:
[window.]document.formName.hiddenName [window.]document.formName.elements[index] [window.]document.forms[index].hiddenName [window.]document.forms[index].elements[index]

Event Handlers
Since a hidden object cannot be seen on a page, no events can be associated with it, and hence there is no support for event handlers for hidden objects.

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The function is a bit tricky. It first encodes the textarea’s value via the escape function and assignes the “escaped” string to a local variable, str. You may recall that after escape, any nonalphanumeric characters are represented in a “%XX” format. The escape sequence “\n” is represented as “%0A” (zero + A) whereas “\r” is converted to “%0D” (zero + D). The entire encoded string is then spliced with the split() method, using either the “%0D%0A” or the “%0A” string as the delimiter. The “trick” is to use the second delimiter (“%0A”) only after the first one (“%0D%0A”) fails to split the text. At the end, each element of the ar array is decoded back by the unescape function. The final array of lines is returned.

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Properties and Methods
Since all methods associated with form element objects emulate events, a hidden object does not have any methods. A hidden object does not feature any event handlers either, but it does have several properties which you can access via JavaScript. The object’s properties are: n defaultValue n name n value These properties are equivalent to those of the text, password, and textarea objects. You can read and set the value property, for example, at any time. The value property can be used to earmark forms submitted to the server for identification by a serverside application.

button, submit, and reset Objects
HTML Syntax
The most precise input event is a button click. submit, reset, and button objects are all buttons which feature identical properties and attributes. The general syntax of a button is as follows:
<INPUT TYPE="button" | "submit" | "reset" NAME="buttonName" VALUE="buttonText" [onClick="handlerText"]>

Although a plain button (<INPUT TYPE="button">) does not have any explicit meaning in HTML, it is extremely useful in JavaScript scripts. A Submit button plays a very important role in all forms that are submitted to a server. A reset button resets the form in which it is located. A button’s style depends on the client’s platform. Macintosh buttons, for example, are different from Windows 95/98/2000 buttons. The only control you have over a button’s appearance is by determining its label, initially defined by the VALUE attribute. Since the button size is solely determined by the label string, you can only modify its width, by padding it with spaces on both ends. This “sophisticated” technique may be very annoying, but it is the only way to explicitly control a button’s size. A Submit button’s syntax differs from plain buttons and reset buttons only in the value assigned to the TYPE attribute "submit". This button is primarily designed for server-side applications. When the user clicks such a button, the form’s data is sent to the URL specified in the <FORM> tag’s ACTION attribute. A Reset button’s TYPE is "reset" and, when clicked, it resets the entire form. A plain button (with no HTML meaning) is defined by assigning "button" to the TYPE attribute.

Event Handlers
All button objects (button, reset, and submit) are associated with a single event, and therefore support only one event handler. This event handler is probably the most important of all form-related event handlers. onClick A button, by definition, has only one designation—to be clicked. Its event handler, therefore, responds only to a click event. Such an event occurs when the user presses the button and then releases it while the pointer is still atop the button. The event does not take place immediately when the user presses the button, in order to give the user a chance to cancel the clicking action by releasing it outside the button area. You can use this event handler also with Submit and Reset buttons. The onClick event handler script is executed prior to performing the built-in action associated with a button. You can use this event, for example, to compute a certain expression and assign its value to the value property of an existing form element object. The general syntax for this event handler is as follows:
<INPUT TYPE=... onClick="handlerScript">

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Starting with Netscape Navigator 3.0 and Internet Explorer 5.0, there is a new option to cancel a click event. This can be done by having the event handler return false, much the same way you cancel a form’s submission or resetting. The canceled action is the one defined by the button (or other objects that support this event handler). Canceling a click event of a Submit button, for example, cancels the form’s submission. Example 20-5 demonstrates the usage of the onClick event handler for invoking a function that handles other elements of the form:
<HTML> <HEAD> <TITLE>Expression evaluator</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function compute(form) { form.result.value = eval(form.expression.value) } // --> </SCRIPT>

Before we discuss this short script, it is essential that you know how the page looks. Figure 20-7 shows the initial appearance.

Figure 20-7. A form with four elements that computes numeric expressions.

The user types any numeric expression (such as 4 + 6 * 15) in the upper left field, and, after pressing Compute, the result appears in the upper right field. The Clear Result button deletes the value in that field. As you can see, the form contains four elements. The first two are text objects—each with its own unique attributes. The first button, named computeButton, uses an onClick event handler to call the compute function with the object reflecting the entire form. The second button provides the only means for clearing the result field (the upper right one). The onClick event handler of this object is a single, immediate statement. Its onFocus event handler creates a read-only field by calling the blur method. Since the entire form object is assigned to compute’s form parameter, the property form.result.value refers to the content of the top right text object, whereas form.expression.value refers to the text inside the first field. The text in the first field is evaluated, and the numeric result becomes its value. If the left field contains the expression 5 * 3 – 2, for example, then eval returns 13, which is placed in the second text object by assigning 13 to its value property.

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Methods
click() A button’s click() method simulates the user’s action of clicking that button. It causes the same action as would a human click, except that the button’s onClick event handler is not triggered. Since their only usefulness is in triggering the onClick event handler, this method is not useful for TYPE="button" buttons. On some platforms, a visual border effect is created when invoking this method. You will probably never need to use this method, but here is an example for your reference:
document.forms[0].elements[0].click()

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Properties
name A button’s name property is an exact reflection of the NAME attribute and is read-only. In order to respond correctly to an event, the onClick event handler function commonly uses this property to figure out which button was clicked. Here is an example of how to reference this property:
var buttonName = document.forms[0].elements[0].name

value A button’s value is the visual label you give the button by assigning it to the VALUE attribute of the <INPUT> tag. All form elements defined by the <INPUT> tag feature this attribute. As far as buttons are concerned, the length of the string determines the button’s size. The VALUE attribute can be more than one word, and should generally be enclosed by quotation marks. The VALUE attribute is reflected in JavaScript by the value property. Although it is unlikely you will ever need to extract the property’s value, its modification may be useful. Suppose you have a single button on a web page, and you want its label to change whenever the user clicks the button. You can implement such behavior by modifying the value property as a response to the click event. Netscape Navigator 3.0 and up and Internet Explorer 5.0 and up enable explicit JavaScript setting of event handlers, so you can even modify the event handler’s script along with the button’s label, to create a “new” button on the fly (without reloading the page). Example 20-6 uses these features to create a simple stopwatch.
<HTML> <HEAD> <TITLE>stopwatch (timer)</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-// set initial values var timerRunning = false var timerID = null // create instance of Date object representing current time

result = "0" + result // convert result to string result = result.toString() // if result is integer if (result.indexOf(".") == –1) // attach ".00" to end result += ".00" // if result contains only one digit after decimal point if (result.length – result.indexOf(".") <= 2) // add a second digit after point result += "0" // place result in text field document.forms[0].display.value = result // call function recursively immediately (must use setTimeout to avoid overflow) timerID = setTimeout("showTimer()", 0) // timer is currently running timerRunning = true } // --> </SCRIPT> </HEAD> <BODY> <FORM> <INPUT TYPE="text" NAME="display" VALUE="" onFocus="this.blur()"> <INPUT TYPE="button" NAME="general" VALUE="start" onClick="start()"> </FORM> </BODY> </HTML>

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Example 20-6. A simple timer with an adjusting button.

Look through the script for a few minutes, and make sure you understand the recursive flow and the handling of time-related features. Since most of the features in this script should be well known to you by now, we’ll just cover the new and difficult ones. Take a look at the following statements from the start() function:
// set the button's label to "stop" document.forms[0].general.value = "stop" // assign the stop function reference to the button's onClick event handler document.forms[0].general.onclick = stop

The first statement adjust the button’s label by assigning it a new value—stop. The second statement assigns a function reference (no parentheses because it is a function reference, not a function call) to the button’s onClick event handler. Notice that the event handler must be specified in lowercase letters. These statements convert the Start button to a Stop button. The following statements from the stop() function convert the Stop button back to a Start button, on the fly:
// set the button's label to "start" document.forms[0].general.value = "start" // assign the start function reference to the button's onClick event handler document.forms[0].general.onclick = start

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Notice that the button is labeled Stop when the timer is running and is labeled Start otherwise. The form in Example 20-6 consists of two elements. The first one is a read-only text object (set via an event handler), whereas the second one is a simple button. Setting the value of a button is somewhat problematic. First, the button size is not adjusted to the new label, so any excess text is dropped on both sides. Another problem is that since the button label is centered, you do not have control over its alignment with other buttons of different form elements. If you initialize a button to a very long string via its HTML attribute (try assigning a string with many spaces to VALUE), you can even run a T-banner on the button!

checkbox Object
HTML Syntax
A check box is a small box that reflects an “on” or “off” state. An HTML check box is a form element that closely resembles a check box on a paper form. If a mark is placed in the box, the current state is considered true. If the check box is unchecked, the current state is false. Check boxes are used a lot in server-based forms. If a box is checked, it is included in the submitted data, whereas if it is empty it is excluded from the data submitted to the server. Since a check box is an individual form element that is not grouped with other elements, you should apply a distinct name to every check box in a form. The user can check or uncheck a text box at any time after the page has been laid out. The general syntax of a checkbox object is as follows:
<INPUT TYPE="checkbox" NAME="checkboxName" VALUE="checkboxValue" [CHECKED] [onClick="handlerText"]>

Both the NAME and VALUE attributes specify internal values which are not displayed on the page. In order to label a check box, you should simply place the label directly after the check box, so it appears as if connected to the object. The following piece of code demonstrates this trick:
Select <P> <INPUT <P> <INPUT <P> <INPUT all the computers used at your business: TYPE="checkbox" NAME="PC">PC TYPE="checkbox" NAME="Mac">Macintosh TYPE="checkbox" NAME="Unix">Unix (X-Windows)

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The output page is the following:

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In order to create a check box that is initially checked, you simply specify the CHECKED attribute, without assigning it any value. Its presence determines the initial state of the check box when the page loads. A JavaScript check box provides a wide variety of functionality, but you should not use it as a regular button (supporting, for example, an onClick event handler); rather, use it only as a yes/no selection interface. You can, however, use the click event to trigger a side-effect statement. It is important to recognize that a check box is primarily a toggle switch. Chapter
Figure 20-8. A form with check boxes.

JavaScript Access
There are basically four ways to reference a checkbox:
[window.]document.formName.checkboxName [window.]document.formName.elements[index] [window.]document.forms[index].checkboxName [window.]document.forms[index].elements[index]

Event Handlers
onClick Since a check box responds to only the click event, the checkbox object does not support event handlers other than onClick. Use this event handler when you want to invoke a function or execute a statement immediately when the user clicks a check box. A click event occurs when the user clicks a check box, regardless if it is checked or empty. Again, be extremely conservative when using this event handler. A check box should be used only for yes/no selection.

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Methods
click() The checkbox object supports only the click() method, which is equivalent to manually clicking the check box. You can use this method to implement a nonmodifiable check box, for example, by clicking the check box whenever the user clicks it, thus always reversing the user’s action and leaving the check box untouched. Here is how to define such a “read-only” check box:
<INPUT TYPE="checkbox" onClick="this.click()">

Another useful implementation of this method is to open an invisible connection between two or more check boxes. Here is an example script that always keeps two check boxes at the same state (checked or unchecked):
<SCRIPT LANGUAGE="JavaScript"> <!-function connectMe(destination) { document.forms[0].elements[destination].click() } // --> </SCRIPT> </HEAD> <BODY> <FORM> <INPUT TYPE="checkbox" onClick="connectMe(1)"> Number 1<BR> <INPUT TYPE="checkbox" onClick="connectMe(0)"> Number 2 </FORM>

Each check box’s onClick event handler invokes the connectMe() function with the target check box’s index as the argument. The second check box’s event handler, for example, calls connectMe() with a 0 value because it is connected to the first check box (its index is 0). The connectMe() function simply emulates a click event for the form element at the given index. The click() method does not trigger the onClick event handler so you should not worry about the script getting into an infinite loop here. You can generalize the connectMe() function to handle an array of check boxes with a for loop.

Properties
checked The check box’s Boolean property checked reflects the button’s current state. A true value means that the specified check box is checked, whereas a false value represents an unchecked box. Of the properties supported by the checkbox object, this is both the simplest and the most useful. You can read and adjust this property at any time, even after layout has been completed.

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Checking and unchecking a check box is accomplished by simply assigning the corresponding Boolean value to the object’s checked property. The following statement unchecks a box:
document.forms[0].elements[0].checked = false

Since a check box can be set to either of the two possible states regardless of the current one, you do not have to use an if statement to determine the current state and then invoke the action accordingly. Be aware that setting the state of a check box by assigning checked is much more efficient than probing the current state and calling the check box’s click() method. Example 20-7 demonstrates the use of the checked property in a simple game. It features both reading the checked property as well as setting it. The objective of the game is to check as many boxes as possible within 20 seconds (20,000 milliseconds). The accumulated number of currently checked boxes is displayed in a text object at the top. When time is over, all check boxes become read-only, and a Start button is placed at the bottom for restarting the game.
<HTML> <HEAD> <TITLE>Checkbox game</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-// assign initial values to global variables var total = 0 var play = false // react to a click in a checkbox (element == clicked checkbox object) function display(element) { // assign instance of Date object representing current time var now = new Date() // if the game has not started yet if (!play) { // game starts now play = true // milliseconds since 1970 for time at beginning startTime = now.getTime() } // if more than 20 seconds have passed since startTime was last set if (now.getTime() – startTime > 20000) { // reject modification (make "read-only") element.checked = !element.checked // terminate function return } // if the clicked check box is now checked if (element.checked) // increment total total++

The global statements in the <HEAD>…</HEAD> portion define the variable total that holds the accumulated number of checked check boxes and the variable play that

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holds a Boolean indication as to whether the game is currently being played (timer is running) or not. Since its statements are all immediate ones, the first script segment to be executed is the one within the <BODY>…</BODY> portion of the page. That script simply prints the HTML interface of the game: a text object, 100 check boxes, a button, and two horizontal rules. All check boxes have an onClick event handler that calls the display() function with the checkbox object itself (this) as an argument. The display() function accepts one argument—the triggered checkbox object. First, an instance of the Date object, representing the time at which the display() function was called, is assigned to the local variable now. The next segment of the function checks if the game is already being played. If not, play is set to true and the number of milliseconds since 1970 is assigned to the global variable startTime. If the difference between the current time and that at the beginning of the game is greater than 20 seconds, the checked property of the calling checkbox is reversed (rejecting the user’s attempts to continue playing after time is out), and the function terminates immediately. The rest of the function deals with a normal event of the user clicking the check box. The total number of check boxes is incremented if the calling check box is currently checked, and is decremented if the check box is currently unchecked. The total number of checked check boxes is assigned to element.form.num.value, the text object’s value property. The restart() function simply resets all global variables and checkbox states to their initial settings.

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Note: Instead of using the click() method to restrict modification of a check box, you should simply return the box’s state to its previous one. That is, set it to true if it was false and vice versa. A general “read-only” check box uses the following syntax:
<INPUT TYPE="checkbox" onClick="this.checked = !this.checked">

defaultChecked A check box definition may include a CHECKED specification to signal that the default state of the check box is “on,” or true. If you do not specify this HTML attribute, it defaults to false. You can access a check box’s default state via JavaScript’s defaultChecked property. You can set defaultChecked at any time, thus overriding the CHECKED attribute. Use the following statement to reset a check box’s state to its default:
this.checked = this.defaultChecked

this is the specific check box you want to reset.

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name A checkbox object’s name property reflects the NAME attribute of the <INPUT> element definition. It is a read-only property, so you cannot modify it. You should be as descriptive as possible when choosing a NAME attribute, especially for server-side applications that access the box’s value only through its name. If your form is primarily for JavaScript processing, you can use the NAME attribute to mimic the functionality of a second VALUE attribute. value The value property initially reflects the VALUE attribute of the <INPUT> definition, but it can be adjusted via JavaScript script at any time. Just like a text object’s string value, the value property represents the checkbox object’s value, and as such is very important for forms processed by the server. By setting the VALUE attribute, you explicitly affect the content of the form submitted to the server. Although you can accomplish many tasks without using the value property, it is sometimes very convenient to use this property instead. The following example demonstrates how to use a check box list to print all operating systems your business uses:
<HTML> <HEAD> <TITLE>Checkbox value property</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function displayList(list) { var total = "" for (var i = 0; i < list.length; ++i) { if (list[i].checked) total += list[i].value + "\r" } if (total == "") alert("No OS selected!") else alert(total) } // --> </SCRIPT> </HEAD> <BODY> <FORM> <INPUT TYPE="checkbox" VALUE="DOS / Windows" NAME="computer">DOS / Windows 3.1x<BR> <INPUT TYPE="checkbox" VALUE="Windows 95" NAME="computer">Windows 95<BR> <INPUT TYPE="checkbox" VALUE="Windows 98" NAME="computer">Windows 98<BR> <INPUT TYPE="checkbox" VALUE="Windows NT" NAME="computer">Windows NT<BR> <INPUT TYPE="checkbox" VALUE="Win2000" NAME="computer">Win2000<BR> <INPUT TYPE="checkbox" VALUE="Macintosh" NAME="computer">Macintosh<BR>

First of all, notice that the form consists of four check boxes and one button. The button’s onClick event handler invokes the displayList() function, passing the computer form element as an argument. Since there are four elements matching that name, this.form.computer is actually an array of four check box elements. A checkbox’s VALUE attribute specifies the operating system associated with that check box. The displayList() function is based on a simple for loop which concatenates the check box values inside the local variable total, inserting new line characters as delimiters. When the loop terminates, total holds a list of all the operating systems that the user had checked. If the value of total is an empty string, the user did not check any of the boxes, and an appropriate message is displayed. Otherwise, the list of the checked operating systems is displayed in an alert box.

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radio Object
HTML Syntax
A radio button provides an interface to select an option among multiple choices. Unlike check boxes which are rectangular in shape, radio buttons are circular. Another difference between the two types of buttons is that radio buttons belong to a group of buttons and are not independent. A group of radio buttons behaves exactly like the station buttons on an old car radio. It has two possible states: one button is pressed (on) or all buttons are released (off). When a button is pressed, the button that was pressed is automatically released. All buttons of the same group share the same name. In fact, the identical name is an indication to the browser to restrict the number of checked radio buttons to one. Clicking a button automatically deselects the currently selected button of the same group. The general syntax of a single radio button is as follows:
<INPUT TYPE="radio" NAME="groupName" VALUE="buttonValue" [CHECKED] [onClick="handlerText"]>

Keep in mind that you should only use this type of button to enable a single selection among multiple choices. The following construct, for example, provides the means to choose a computer type out of three options:
What computer do you most often use: <P>

When implemented correctly within a form, this group of elements appears as outlined below:

Figure 20-9. A form with radio buttons.

JavaScript Access
Although a radio button is a simple form element, its reference is an unusual one. You can reference a radio button group via one of the following expressions:
[window.]document.formName.radioGroupName [window.]document.formName.elements[ElementIndex] [window.]document.forms[FormIndex].radioGroupName [window.]document.forms[FormIndex].elements[ElementIndex]

As explained at the beginning of this chapter, elements with the same names are combined and indexed into a single array. Therefore, the above references are actually arrays, where each element is a single radio button. Hence, the four ways to reference a specific radio button are as follows:
[window.]document.formName.radioGroupName[ButtonIndex] [window.]document.formName.elements[ElementIndex][ButtonIndex] [window.]document.forms[FormIndex].radioGroupName[ButtonIndex] [window.]document.forms[FormIndex].elements[ElementIndex][ButtonIndex]

You can look up the number of radio buttons in a group by accessing the group’s LENGTH attribute. For more details, check the section about the name property of the text object. When a radio button group consists of only one radio button, it is not considered a group. Therefore, you cannot access the array, and references such as the following ones are not valid:
[window.]document.formName.radioGroupName[index] [window.]document.formName.radioGroupName.length

Instead, you must access the radio button directly, without an array:

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[window.]document.formName.radioButtonName

The difference in referencing a single button and a multiple-button group complicates scripts quite a bit. If the radio buttons are created with plain HTML, you obviously know if the number of radio buttons is greater than one and access them accordingly. If the radio buttons are automatically generated by a script, then you have to add a counter that will indicate if the number of buttons is greater than one.

Event Handlers
onClick A radio object supports only one event handler—the onClick event. When the user checks (fills) a radio button, a click event occurs, and the specified statements are executed. See the section on the checkbox object for more details, syntax, and examples.

The function lay() prints a form with a given number of radio objects. Notice the very important technique to encode the greater than character. This character (>) is assigned to the variable gt via the unescape() function. You may recall from Chapter 3 that Netscape uses a different HTML comment than other browsers. While standard browsers terminate a comment with a greater than character (>), Navigator uses a three-character string (-->) instead. Therefore, if a greater than character is really placed in the script, it terminates the comment that hides the script from browsers that do not support JavaScript (specified by the <SCRIPT> tag). When creating a JavaScript-powered web page, you should be sure to use this technique whenever possible, especially to close HTML tags that are printed via document.write. You can also use less than operators instead of greater than operators, simply by reversing the order of the operands of a conditional statement. If your page is based on JavaScript, though, you may choose to disregard other browsers because they probably won’t be able to display the page anyway. Nevertheless, if you are using JavaScript only to add some special effects, you should use this technique to make it clearly viewable with any browser, even those that do not feature JavaScript. Yahoo, for example, used this technique to create a “Yahoo Remote” for its page. The button that launched this JavaScript device was printed by a script, so browsers without JavaScript didn’t even see the button. Had the greater than character not been encoded, the whole page (not just one button) would have been scrambled for users without JavaScript-enabled browsers. Back to the lay() function. It is important to name the form for later access—animation is chosen. A simple for loop is executed to print num (the parameter) radio objects named lamps (belong to the same group). Notice that all HTML tags are printed in the following fashion:
document.write("<TAG ATTRIBUTES" + gt)

As explained before, the value of gt is a greater than character (a one-character string). See the preceding section for a complete explanation on the reason for this encoding. The second function, animate(), is responsible for the actual animation. A global variable, current, is already defined and initialized to 0. The function’s main task is to invoke the click() method associated with the radio object of index current. Note

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that all radio objects are elements of a unique array—document.animation.lamps. The second statement handles the boundary case when the checking loop needs to advance from the last button of the row to the first one (wraparound). The expression current == document.animation.lamps.length – 1 evaluates to true when the value of current is equal to the index of the last element in the document.animation.lamps array (the last radio button). In this case, current is set to 0, forcing the first button on the row to be checked next. In all other cases, the value of current is incremented, advancing the checked radio button. Note that when a radio button belonging to a group is clicked, any previously selected button is deselected. The last statement of the function recursively calls itself after a pause of speed milliseconds (speed is a global variable).

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Properties
checked The checked property evaluates to a Boolean value. If its calling radio object is checked (highlighted), the value of checked is true; otherwise, it is false. Due to the structure of a radio object group, the checked property of a single radio object in a group is always true. You can set the value of checked in order to modify the radio object’s display. Since all buttons of a group can be deselected, setting a checked property to false simply deselects the calling radio object, without causing any side effects to the other buttons. Checking an empty radio object, however, does deselect any previously highlighted member of the group. See the section on the checkbox object for full coverage of this property. defaultChecked The defaultChecked property reflects the HTML CHECKED attribute. Refer to the section on the checkbox object for further details. name The name property initially reflects the NAME attribute in the HTML definition. See the section on the checkbox object’s name property for a complete explanation. value The value property initially corresponds to the VALUE attribute of a radio button HTML tag, but it can be adjusted to any valid string. Once again, refer to the section on the checkbox object for the syntax and a full description.

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select Object
HTML Syntax
Scrolling menus are a flexible means of input you can use on your forms. The <SELECT> tag is used to create various types of scrolling menus. This is the common way to enable the user to select an option from a list. The <SELECT> tag is specified by a <SELECT></SELECT> pair. You should always specify the menu’s name in the following fashion:
<SELECT NAME="anyName">

It is preferred to use a name that meets the JavaScript identifier naming conventions. As you might expect, the interior of the <SELECT>…</SELECT> section includes the list’s options. An option is specified in the following form:
<OPTION VALUE="optionValue">optionText

The following image shows two identical menus; the first is in its initial state and the second one is opened by the user:

Figure 20-10. A form with two menus, one closed and one open.

You can use the SELECT attribute (no value is necessary) to automatically select a default option when the page loads. For example, the following script creates a menu, where the default option (which is also the value that appears highlighted in the box) is “Sun”:
<SELECT NAME="comp"> <OPTION VALUE="http://www.microsoft.com/">Microsoft <OPTION VALUE="http://home.netscape.com/">Netscape <OPTION VALUE="http://www.sun.com/" SELECTED>Sun </SELECT>

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The text in the box is not the VALUE attribute specified inside the <OPTION> tag, but is rather the string placed outside the <OPTION> tag. The string has no effect other than being the menu label. The main advantage of this menu is that, due to its pop-up configuration, it does not take up much space on the page. Its disadvantage is that the user can select only one option. To work around this deficiency, you can specify the MULTIPLE attribute, but, since all options are laid out instead of popping up, you lose the space advantage. Here is a simple example:
<SELECT NAME="comp" MULTIPLE> <OPTION VALUE="http://www.microsoft.com/">Microsoft <OPTION VALUE="http://home.netscape.com/">Netscape <OPTION VALUE="http://www.sun.com/" SELECTED>Sun </SELECT>

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You can select multiple options by holding the Shift button down and selecting or deselecting options from the menu. The Control key can also be pressed in order to select each option individually. These keys are commonly used in all major operating systems. The complete syntax for the select object is as follows:
<SELECT NAME="selectName" [SIZE="integer"] [MULTIPLE] [onBlur="handlerText"] [onChange="handlerText"] [onFocus="handlerText"]> <OPTION VALUE="optionValue" [SELECTED]>textToDisplay […<OPTION VALUE="optionValue" [SELECTED]>textToDisplay] </SELECT>

You should recognize all attributes except for the event handlers, which are explained later.

JavaScript Access
There are several ways to reference a select object:
[window.]document.formName.selectName [window.]document.formName.elements[index] [window.]document.forms[index].selectName [window.]document.forms[index].elements[index]

You can access a specific option in a select object by appending an options property to its reference. This property is actually an array of options starting at index 0. You can reference, for example, the first option of a select object in the following fashion:
[window.]document.formName.selectName.options[0]

The options array is discussed later as one of the select object’s properties.

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Event Handlers
onBlur A blur event occurs when a select object loses focus. The onBlur event handler’s script executes upon a blur event. There are a few ways to generate a blur event: n Select an option from the list and then click outside the select object, either on the page’s body or in another form element. n Select an option from the menu and then send the browser’s window to the background. The new window you focus on may be a different browser window or a window associated with any foreign application. This event handler is not very useful, because it relies on the user to remove focus from the object, which is not that intuitive. onFocus A focus event associated with a select object occurs when the user gives it focus; that is, when the user attempts to select an option from the menu but before the menu pops up or modifies the currently selected option. The onFocus event handler responds to a focus event. When the user clicks somewhere inside the select object, you can, for example, pop up an alert box with user instructions for that particular menu. The following source demonstrates this suggestion:
<FORM> <SELECT NAME="comp" onFocus="alert('Simply select the desired home')"> <OPTION VALUE="http://www.microsoft.com/">Microsoft <OPTION VALUE="http://home.netscape.com/">Netscape <OPTION VALUE="http://www.sun.com/" SELECTED>Sun </SELECT> </FORM>

onChange A change event associated with a select object occurs when the user changes the selected option. Unlike change events associated with other objects, the select object does not have to lose focus in order to generate the event. The onChange event handler executes when a change event occurs. The onChange event handler is commonly used for exchanging information with the user. We will demonstrate this event handler in later examples, when we discuss the select object’s methods and properties.

Methods
blur, focus The only explicit methods of the select object are blur() and focus(). See the listings for the text object at the beginning of this chapter for complete coverage of these methods.

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Properties (select Object)
The select object features various properties, but the most useful are actually properties of the options array, as you will find out later. In this section we discuss only the properties that belong directly to the select object. length You can access the number of options in a select object through the length property. Since this value is also referenced as the length property of the options array, you have the freedom to choose which length to use. Since length is known to be a standard property of all arrays in JavaScript, we personally prefer referencing length via the options array. Netscape Navigator accepts any of the following references to the number of options in the select object:
selectName.length selectName.options.length selectName.options.options.length selectName.options.options.options.length

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Microsoft Internet Explorer accepts only the first two, but, since the third one does not make any sense, you should avoid using it anyway. The value of the length property should not be assigned by a script. It is dynamic, so it changes whenever an option is added to the corresponding select object. name The name property reflects the NAME attribute of the <SELECT> tag. You can modify this property freely. Adjusting it via a script overrides the initial HTML setting. The following script segment shows how to display the name property (“products”) via an alert dialog box:
<SCRIPT LANGUAGE="JavaScript"> <!-function sayName(selectObject) { alert(selectObject.name) } // --> </SCRIPT> <FORM> <SELECT NAME="products" onChange="sayName(this)"> <OPTION VALUE="sny">Sony <OPTION VALUE="jvc">JVC <OPTION VALUE="tsh">Toshiba </SELECT> </FORM>

options You can reference the options in a select object by the options object. Generally speaking, this array contains an entry for each option (<OPTION>) in a select object

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(<SELECT>). Suppose the first element of the first form in a document is a select object with three options. These options can be referenced in JavaScript as:
document.forms[0].elements[0].options[0] document.forms[0].elements[0].options[1] document.forms[0].elements[0].options[2]

As always, the length of the array, which is equal to the number of options in the given select object, is stored in the array’s length property. In the preceding examples that would be:
document.forms[0].elements[0].options.length

Elements of the options array are read-only. Although it does not generate a JavaScript error, assigning a value to any of the elements has no effect. The bare-bones selectObject.options reference evaluates to the full HTML syntax used to create the specified selectObject. Note that you can also access the properties of the select object as if they were direct properties of the options array. selectedIndex The selectedIndex property is an integer specifying the index of the selected option in a select object. Options in a select object are indexed in the order of definition, i.e., in the same order they are entered in the options array. You can also set the value of selectedIndex in a script to immediately update the state (the selected option) of a select object. The selectedIndex property is not useful with multiple select objects because it can only refer to the first selected option in the list. You can work around this problem by using the selected property of the options array and a simple loop. See the listings for this property for further details. Example 20-9 and Figure 20-11 demonstrate the selectedIndex property. Although we have not discussed the value property yet, its role is obvious. If needed, refer to its description later in this chapter.
<HTML> <HEAD> <TITLE>URL option</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function loadPage(list) { location.href = list.options[list.selectedIndex].value } </SCRIPT> </HEAD> <BODY> <FORM> <SELECT onChange="loadPage(this)"> <OPTION VALUE="http://www.cnn.com/">CNN

When the user selects an option in the select object, the loadPage() function is invoked by the onChange event handler, passing the select object as an argument. The URL associated with each option is stored as the option’s VALUE attribute, or value property in terms of JavaScript. The selected option is list.options[list.selectedIndex] because list.selectedIndex is an integer representing the index of the selected option. The value property is used to access the URL of the selected object, which is then assigned to location.href, in order to load that page to the browser window. You may prefer to use a button in place of the onChange event handler:
<HTML> <HEAD> <TITLE>URL option with button</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function loadPage(list) { location.href = list.options[list.selectedIndex].value } </SCRIPT> </HEAD> <BODY> <FORM>

Example 20-10. Another version of the URL picker—this one uses a button.

The only difference between Example 20-10 and Example 20-9 is that in Example 20-10 the event handler is attached to a button object, while in Example 20-9 it is attached to a select object. In order to avoid changing the function, we have modified the argument handed to the function to keep passing the select object. The expression this.form references the form and elements[0] references the select object which is the first element in the form.

Properties (options Array)
An element in the options array reflects a select object’s option defined by the <OPTION> tag in HTML. Properties of the options array are properties of specific options in a select object. defaultSelected The defaultSelected property evaluates to a Boolean value. If the specified option is defined with a SELECTED attribute (<OPTION…SELECTED>), the value of defaultSelected is true. Otherwise, it is false. defaultSelected initially reflects whether the SELECTED attribute is used within an <OPTION> tag. Setting the defaultSelected property via a script overrides the initial HTML setting. In a select object without a MULTIPLE specification you can only have one option selected by default. Therefore, setting the defaultSelected property of a given option to true clears any previous default selections, including those set with SELECTED. Nevertheless, if you set defaultSelected in a select object defined with the MULTIPLE attribute, previous default selections are not cleared. index The index property of a single option in a select object is the number identifying the position of the object in the selection list, starting from zero. Under normal circumstances, there is no justification for the existence of the index property, because in order to reference an option, you need to know its index:
document.selectObject.options[indexValue]

and when you reference the index property, you supposedly know the index already:
document.selectObject.options[indexValue].index

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length See the listings for the length property in the preceding section on select objects. selected The selected property is a Boolean value specifying the current selection state of an option in a select object. Its general syntax is as follows:
selectName.options[index].selected

The selected and defaultSelected properties are very useful. Suppose you want to create a button by which the user can reset the select object. Using the Reset button is not desirable because it resets the entire form, not just the select object. You can solve the problem by using a simple JavaScript function to revert the select object to its default state. Here is the function that the button should invoke:
<HTML> <HEAD> <TITLE>Reset select object</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function setDefault(selectName) { for (var i = 0; i < selectName.options.length; ++i) { selectName.options[i].selected = selectName.options[i].defaultSelected } } // --> </SCRIPT> </HEAD> <BODY> <FORM> <SELECT NAME="myMenu" MULTIPLE> <OPTION> First option <OPTION SELECTED> Second option <OPTION> Third option <OPTION> Fourth option <OPTION SELECTED> Fifth option </SELECT> <INPUT TYPE="button" VALUE="reset menu" onClick="setDefault(this.form.myMenu)"> </FORM> </BODY> </HTML>

Example 20-11. A simple function resets a select object.

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If an option in a select object (selectName.options[index]) is selected, the selected property evaluates to true. Otherwise, it evaluates to false. You can set this property at any time, which immediately affects the display of the select object.

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The setDefault() function works with any type of select object, whether it is a multiple one or not. It simply loops through the options of the select object and sets the selected property of each to its defaultSelected property, reverting all options to their default selection state. The value handed to the function is the select object, referenced as this.form.myMenu.

þ

Note: Microsoft Internet Explorer does not fully support the select object. The entire discussion is based on Navigator because it simply does not work for Microsoft’s browser. As you can see, Microsoft has focused on the user interface of its browser but did not pay much attention to script-level access via JavaScript. The average web surfer does not feel the disadvantages because web content providers use only features that are supported by both browsers.

text There is no HTML attribute that defines the option’s label. Take a look at Example 20-11. The strings “First option,” “Second option,” and so forth are not specified in any HTML attribute as you could expect. They are simply appended to the <OPTION> definitions. The text property can be set at any time, immediately affecting the display. Example 20-12 demonstrates the use of this property. When the user selects an option from the list, that option is automatically placed at the top of the list, shifting all other options downwards. See the listings for the value property if this is not completely clear.
<HTML> <HEAD> <TITLE>Swapping options</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-var choiceIndex = –1 function swap(opt1, opt2) { var tempText = opt1.text var tempValue = opt1.value var tempDefault = opt1.defaultSelected opt1.text = opt2.text opt1.value = opt2.value opt1.defaultSelected = opt2.defaultSelected opt2.text = tempText opt2.value= tempValue opt2.defaultSelected = tempDefault } function shift(selectName) { if (choiceIndex == –1) { choiceIndex = selectName.selectedIndex swap(selectName.options[choiceIndex], selectName.options[0])

Example 20-12. You can swap options in a select object by swapping their values.

When the user selects an option from the select object, the top option is swapped back to its original position and then the selected option is swapped with the top option that the first swap yielded. At first, when the page loads and the select object has not been through layout yet, the global variable choiceIndex is initialized to –1. Since the index of an option in a select object is always non-negative, –1 is used to indicate that this is the first execution and that the current state of the select object is its initial one. The swap() function accepts two options associated with a select object and simply swaps them by swapping their three properties: n text n value n defaultSelected You cannot directly swap two options because the options array is read-only. The shift() function accepts one argument—a select object. If the value of choiceIndex is –1 then choiceIndex is assigned the index of the selected option, and the selected option is swapped with the option at the top of the list (index is 0). Since choiceIndex is a global variable, its value will stay intact until the next invocation of the shift() function. When the value of choiceIndex is not –1, the function runs a different set of statements. First, it swaps the option at the top of the menu with option number choiceIndex. Since choiceIndex has not been set yet during the current execution of the function, it holds the index of the option that the user selected

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on the previous round. That is, the function simply returns the select object to the state that preceded the previous function’s execution. The next two statements are identical to those executed when the value of choiceIndex is –1. It is unnecessary to specify these statements twice. Instead, you may conditionally execute the first statement (if choiceIndex is not –1) and then unconditionally execute the other two. value The value property initially reflects the VALUE attribute of an <OPTION> definition. You can override the initial setting at any given time by assigning this property a value. We have seen this property in action before. For working examples, see the listings for the defaultSelected property of the select object and the preceding listings for the text property.

The Option Object—Adding Options Using the Option Constructor
JavaScript for Netscape Navigator enables you to explicitly create options via the Option constructor. Options you create are instances of the Option object. The syntax for creating an option is as follows:
var optionName = new Option([optionText, optionValue, defaultSelected, selected])

Here is a brief explanation of the arguments you should normally hand to the constructor function: n optionText—a string representing the option’s text property n optionValue—a string representing the option’s value property n defaultSelected—a string representing the option’s defaultSelected property n selected—a string representing the option’s selected property See the listings for each of these properties for more details. You can add an option to an existing select object in the following fashion:
selectName.options[index] = optionName

After you create an option and add it to a select object you must refresh the document. You can do that via JavaScript using the following statement:
history.go(0)

You can also refresh a document using the browser control options. Either click in the Location box and press Enter, or select Refresh from the View menu. You can delete an option from a select object by assigning it a null value. The general syntax is as follows:
selectName.options[index] = null

Once again, you must refresh the document via history.go(0) in order to see the updated appearance of the select object. An option created as an instance of the Option object includes the same properties as an option associated with a select object.

When the page first loads, two select objects are created. The first one includes three options and is not modified at any stage. The second one is a dynamic select object, i.e., its options and length change. The script in the <HEAD>…</HEAD> portion of the page defines a two-dimensional array. The first index is of the first select object (an integer from 0 to 2). The second index is an integer between 0 and the index of the last option of the second select object. The number of options in the second select object depends on the option selected from the first select object. Sounds difficult, but it’s not! First you must understand what the script does. When the user selects an option from the first menu (select object), the second select object is updated with the corresponding option list. For example, if the second option in the main menu is selected, the script uses its index (1) to determine the array of elements (menu[1]) that should constitute the options of the second select object. For the sake of the example, we used text properties to identify the structure (1-1, 0-3, etc.). When the user first loads the page, the first select object is laid out according to its HTML definition, while the second select object is generated via a JavaScript for loop which iterates over the menu[0] array and writes out its elements. The first select object includes an onChange event handler that invokes the updateSub() function, passing, as arguments, the form and the selected index. The function is based on two loops. The first one deletes the current options of the second menu by assigning each a null value. The second loop adds new options to the second menu, according to the elements of the selected menu array. Notice that we did not add properties for all options, but rather only for the first option of each menu array (a value, a default property, and a defaultSelected property). The elements of the menu array are obviously instances of the Option object. The document is refreshed by history.go(0). An instance of the Option object has the same structure as an element of a select object’s element array. Both objects include the same properties and none have methods.

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fileUpload Object
HTML Syntax
A file upload element of an HTML form enables the user to supply a local file as input. This feature was implemented in Navigator 4.5 and Internet Explorer 5.0. Generally speaking, its syntax is as follows:
<INPUT TYPE="file" NAME="fileUploadName">

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Note that fileUploadName is not the name of the file, but rather the name of the form element that enables the user to supply the file.

Properties
The fileUpload object combines two meaningless properties, name and value. name The name property initially reflects the NAME attribute of the HTML definition. You may set its value at any time. value value is a read-only property that reflects the current value of the file upload element’s field—the name of the file to upload.

Summary
You deserve a reward if you had the motivation to read through this lengthy chapter. I believe that you have already been rewarded with the knowledge to implement interactive forms including their event handlers. Forms are a convenient means of

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collecting user’s input without producing annoying dialog boxes. Some form elements such as text objects and text areas also come in handy when you need to display changing output, such as a clock or a banner. Don’t worry if you do not remember every single method and property of the objects discussed in this chapter. This book will serve as an easy-to-use reference when you actually need to create a JavaScriptpowered form. At this point, you should know the basics of and how to use form elements in JavaScript, and the properties, methods, and event handlers of text objects and regular buttons.

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Links, Anchors, and Image Maps
Defining a Link
Here is the general HTML syntax for a link:
<A HREF="locationOrURL" [NAME="anchorName"] [TARGET="windowName"]> linkText </A>

The HREF attribute defines the document or anchor to which you are linking. The NAME attribute specifies a tag that becomes an available hypertext target within the current document. If this attribute is present, the link object is also an anchor object. The TARGET attribute specifies the window where the linked document should load (e.g., a name of a window, a name of a frame, a special literal frame name such as _top, _parent, _self, and _blank). You can create a link with a plain HTML tag as shown above, but you can also use JavaScript. The following script segment demonstrates how to create a link with JavaScript:
document.write(linkText.link(hrefAttribute))

The preceding statement uses the link() method of the String object. See Chapter 14, Handling Strings, for instructions on using this method. Another obvious way to create a link with JavaScript is by printing the plain HTML syntax via document.write in the following form:
document.write('<A HREF="locationOrURL" [NAME="anchorName"] [TARGET="windowName"]>linkText</A>')

Although this syntax works on a JavaScript-enabled browser, it contains several flaws that will spoil the appearance of the page when viewed with a browser that does not support JavaScript. The problem is that greater than (>) characters in a script

The first NAME attribute specifies the name of the image map that contains various areas, whereas the second one specifies the name of a single area in the image map. An image map can consist of any number of areas. The COORDS attribute specifies the coordinates of a specific area in a map. This attribute consists of either four coordinates, if the area is defined as a rectangle (an x and y for the top left and bottom right corners), or three coordinates, if the area is a circle (x and y for the center and the radius). HREF specifies the URL of the document to load when the user clicks the area. Any area of the image map that does not have this attribute does not act as a hypertext link. You can use any scheme (protocol) for the location, including a JavaScript statement via javascript:. The SHAPE attribute determines the shape of an area in an image map. If not specified, the shape defaults to rect. TARGET specifies the window or frame in which the destination document is loaded. See the section above on link definition for additional information regarding this attribute.

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links Array
The only way to reference a link object in your code is by using the links array. This array is similar to the forms array. It is also a property of the document object, so you can access this array in the following fashion:
[window.]document.links

This links array contains an entry for each link (<A HREF="…">) and area (<AREA HREF="…">) object in the document. Suppose you have a document containing three links. You can reference these links using the following syntax:
document.links[0] document.links[1] document.links[2]

These references can equally apply to a document with three area objects (within a single or multiple image maps), as well as for a document with two links and one image map area. The total number of qualifying links and image map areas is reflected by the length property of the array:
document.forms.length

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This is probably the most useful link-related property. Elements in the links array are obviously read-only, but assigning a value to a given element does not cause an error—it simply doesn’t work.

anchors Array
You can reference the anchors in your code via the anchors array. This array contains an entry for each <A> tag containing a NAME attribute in a document, in source order. If a document contains three named anchors, for example, these anchors are reflected as document.anchors[0], document.anchors[1], and document.anchors[2]. You can use this array in one of two ways, exactly like you use the links array or the forms array:
document.anchors[index] document.anchors.length

Each element of the document.anchors array holds a null value and does not feature any methods or properties. The value of document.anchors[0], for example, is null, regardless of whether there are anchors in the document. The anchors array does not play a role in any script, but its length property occasionally does. If you use a systematic naming scheme for all anchors in a document, you can take advantage of the length property (document.anchors.length). Suppose a document contains a variable number of anchors. You can name these anchors anchor0, anchor1, anchor2, and so forth, or, alternatively, test0, test1, and test2, if you so desire. The latter anchors would use the following syntax:

Take a look at the following function:
function goAnchor(num) { if (num >= document.anchors.length) alert("Anchor does not exist!") else location.hash = "test" + num }

This function accepts an existing anchor’s index and scrolls the page to that anchor by assigning its name to the location.hash property. It also demonstrates a possible usage of the anchors array, or its length property, to be exact. See Chapter 17, URLs and JavaScript, for details on the location.hash property.

Link and Area Event Handlers
Anchors do not feature any event handlers, but links and image map areas do provide them. Some of the event handlers are very useful, so it is important that you know how to use them.

Calling Event Handlers Explicitly
Since the document.links array holds an entry for every link and image map area in a document, it is fairly straightforward to explicitly call an event handler for a specific link or area by assigning it to the corresponding array element. You can use the following statement to explicitly call the event handler doSomething for the first link or area (whichever is first in source order):
document.links[0].onmouseover = doSomething

onClick
You can add an onClick event handler in the following fashion:
<A … onClick="validJavaScriptCode"> <AREA … onClick="validJavaScriptCode">

A click event associated with a link object or an area object occurs when the user clicks the content of the link, which can be plain text, an image, and so forth. In the case of an area object, the link content is (usually) a portion of an image. The JavaScript statements you specify for the onClick event handler are executed prior to loading the URL defined by the HREF attribute. They can be used to do any last-minute preparations. Usually, you simply want a link or image map area to execute some JavaScript code when the user clicks it. You can accomplish this task by simply using a URL based on a

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javascript: scheme with the HREF attribute. When the user clicks it, the following link calls a function named register:
<A HREF="javascript:register()">Register Profile</A>

If you wish to create a link that does not respond at all to a link but still enables onMouseOver and onMouseOut event handlers, you can use the void operator (compatible with Netscape Navigator 3.0 and above and Internet Explorer 5.5 and above):
<A HREF="javascript:void(0)">Register Profile</A>

onMouseOver
A mouseOver event occurs each time the mouse pointer moves into an object or an area from outside that object or area. Upon this event, the onMouseOver event handler executes JavaScript code. The syntax of a general script to do this is as follows:
<A … onMouseOver="validJavaScriptCode"> <AREA … onMouseOver="validJavaScriptCode">

Instead of having its URL displayed by default in the status bar whenever the user places the mouse over it, this event handler is often used to display a message associated with a link. (You must return true from the event handler.) The following link, for example, displays “Cool shareware stuff” in the status bar when the user places the mouse over the link labeled “Jumbo”:
<A HREF="http://www.jumbo.com" onMouseOver="window.status = 'Cool shareware stuff'; return true">Jumbo</A>

Alternatively, you may choose to explicitly return a true value by adding a statement to the event handler code.

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If the mouse moves from one area of a client-side image map to another, it is the onMouseOver event handler of the destination area (the one you are moving to) that is being triggered.

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onMouseOut
A mouseOut event occurs each time the mouse pointer leaves an area within a client-side image map or a link from inside that area or link. The onMouseOut event handler executes JavaScript code upon this event. If the mouse moves from one area to another in a client-side image map, the onMouseOut event handler of the source area is triggered, and the onMouseOver event handler of the destination area is triggered. If you want an area to use the onMouseOut or the onMouseOver event handler, you should specify the HREF attribute of the <AREA> tag. Nice effects can be achieved by using onMouseOver and onMouseOut together. Upon placing the pointer over a link or an image map area, for example, you can display a message in the status bar, and then, instead of waiting for an arbitrary number of seconds, you can delete it immediately upon the removal of the mouse pointer. The following code demonstrates how to implement such an effect:
<A HREF="http://www.jumbo.com" onMouseOver="window.status = 'Cool shareware stuff'; return true" onMouseOut="window.status = ''; return true">Jumbo</A>

Note that a similar effect can be achieved with an image map area instead of a link. Example 21-1 demonstrates both onMouse event handlers, for a client-side image map. The example prints “HTML.” When the user clicks the “H” area, the word “Hyper” is displayed in an alert box. When he or she clicks the “T” area, “Text” is displayed, and so forth with “Markup” and “Language.” When the mouse pointer is over a certain letter, the corresponding word (“Hyper,” “Text,” “Markup,” “Language”) is displayed in the status bar. When the mouse pointer is removed from the image map, the status bar is blanked. Now, take a look at Example 21-1 to find out how this works.
<HTML> <HEAD> <TITLE>Client-side image map</TITLE> </HEAD> <BODY BGCOLOR="#ffffff"> <IMG SRC="img21-1.gif" HEIGHT=69 WIDTH=214 ALT="HTML" BORDER=0 USEMAP="#html_map"> <MAP NAME="html_map"> <AREA NAME="H" COORDS="0, 0, 55, 69" HREF="javascript:alert('Hyper')" SHAPE="rect" onMouseOver="window.status = 'Hyper'; return true" onMouseOut="window.status = ''; return true"> <AREA NAME="T" COORDS="56, 0, 101, 69" HREF="javascript:alert('Text')" SHAPE="rect" onMouseOver="window.status = 'Text'; return true"

Except for the last one, all attributes are self-explanatory. USEMAP specifies the name of the image map definition (<MAP>) for the image, preceded by a hash character. The image map definition itself uses the following opening tag:
<MAP NAME="html_map">

You should set the name of the map as shown above, or the image won’t be able to refer to it. There are basically four areas in this image map, one for each letter of the “HTML” banner. Since, except for the triggered message and coordinates, all four areas are alike, only one will be discussed here. Let’s take a look at the last area:
<AREA NAME="L" COORDS="161, 0, 214, 69" HREF="javascript:alert('Language')" SHAPE="rect" onMouseOver="window.status = 'Language'; return true" onMouseOut="window.status = ''; return true">

First, notice that this area is defined as a rectangle (SHAPE="rect"). The x and y coordinates of the upper left corner are 161 and 0, respectively. Coordinates are measured in pixels starting at (0, 0), the first pixel in the image. The x and y coordinates of the bottom right corner are 214 and 69, respectively, which are the HEIGHT and WIDTH attributes of the image. When the user places the mouse pointer over this area, the status bar displays the string “Language” (for “L (the string is assigned to the status ”) bar property), and the Boolean value true is returned. When the user removes the mouse from this area, the status bar is blanked (assigned an empty string). Since the onMouseOver event handler of the destination area immediately overtakes the

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onMouseOut operation of the source area, the user usually won’t notice this interim state when moving the pointer to one of the adjacent image map areas. When the user removes the mouse pointer from this area to outside of the entire image, the status bar is cleared and remains empty, until either a different value is explicitly assigned to the status bar or the mouse moves back into the image map area. When the user clicks this area, the location specified in HREF is loaded, and since the value of this attribute is a JavaScript statement preceded by the javascript: scheme, it does not load a new document but rather pops up an alert dialog box with the string Language.

Link and Area Properties
Location-Equivalent Properties
Most of the properties that belong to a link or an area object (document.links[index]) are associated with the URL of the HREF definition. These are all properties of the window.location object, to which a full chapter is dedicated. Only a short description of each property will be given here. The only way to reference a property of a link or an area object is to use the following syntax:
document.links.propertyName

Note that links do not have any methods. Here is the equivalence list between the properties of document.links and those of window.location: n hash specifies an anchor name in the URL. n host specifies the hostname:port portion of the URL. n hostname specifies the host and domain name, or IP address, of a network host. n href specifies the entire URL. n pathname specifies the url-path portion of the URL. n port specifies the communications port that the server uses for communications (80 by default). n protocol specifies the beginning of the URL, including the colon (also known as the scheme). n search specifies a query.

target
The target property initially reflects the TARGET property of the <A HREF="…"> or <AREA HREF="…"> HTML definitions. Setting the value of this property overrides the initial definition. The target property (document.links[index].target) cannot be assigned a JavaScript expression or variable.

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Referring Documents
When a user clicks a link in one document and causes a new page to load in the same or another window, the calling document is known as the referring document. When a user arrives at a page via a link from another page (and not from his or her bookmark, history list, friends, etc.), the URL of the referring page is reflected in the new page as a read-only property, document.referred. A referring document is also called the source document, whereas the new page the referrer document links to is known as the destination document. The property document.referred is very useful for tracking a user’s movement inside a large site you maintain by yourself. It is not helpful at all if the user surfs to foreign pages, so, unless Playboy provides a link to your page, you won’t be able to greet the user with a message such as “Naughty boy—Playboy is for grown-ups only!” Suppose you have a page on your site, C.html, that is linked from two different pages, A.html and B.html. Let’s say A.html includes important copyright information on a shareware product available on C.html, and B.html provides important instructions on installing the software package. You can use the following JavaScript script on C.html to check where the user came from and to print a link to the third page:
<SCRIPT LANGUAGE="JavaScript"> <!-- begin JavaScript *immediate* script // assign greater than character without literally displaying it var gt = unescape("%3E") if (document.referred.indexOf("A.html") == –1) document.write('<A HREF="A.html"' + gt + 'Copyright notice</A' + gt) if (document.referred.indexOf("B.html") == –1) document.write('<A HREF="B.html"' + gt + 'Installation instructions</A' + gt) // --> </SCRIPT>

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This script prints links to the documents the user has not come from. It prints A.html if the user has not come from A.html, B.html if the user has not come from B.html, or both links if the user has apparently arrived from a different document or not from a link at all.

Summary
In this chapter, we discussed the link, area, and anchor objects. You learned that the only way to reference one of these objects is by either document.links or document.anchors. There are three important concepts you should remember from this chapter: n The javascript: protocol specifies a JavaScript statement for the HREF attribute of a link or image map area. n The onMouseOver event handler of links and image map areas can execute a JavaScript statement. When the user places the mouse pointer over the link, it is

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an indication of his or her interest in it, so displaying related information in the status bar may be very helpful in such a scenario. The onMouseOut event handler is also an attribute of a link or image map area. Its usage is similar to that of the onMouseOver event handler.

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Implementing Cookies
Maintaining State
When you create a web site, you expect the user to load HTML documents, view them, navigate from one page to another, etc. Occasionally, it is important to enable the web page to maintain a state. That is, the page “remembers” certain actions executed by the user during previous sessions. A classic example of maintaining a given state is a shopping cart application, as implemented, for example, in an online store. The user travels from one product review to the other via simple HTML links. When he or she comes across an interesting product, clicking a button puts the selected product’s data in a “shopping cart.” The shopping cart, which is sometimes displayed visually on the page, is basically a name for a storage mechanism. Since it is not possible to store the data for each user on the server, the data is kept on the client side, in what is called a shopping cart. Cookies are a general mechanism that server-side applications (such as CGI) and client-side JavaScript scripts can use to store textual data on the client side for the purpose of retrieving it later. The name “cookies” does not have any technically significant meaning; it is simply a name the inventors thought was cool. Cookies are tidbits of information stored in a browser-dependent format on the client machine. Netscape Navigator, for example, holds all cookies in a regular text file named cookies.txt (in the directory where Navigator is installed), whereas Internet Explorer stores cookies in multiple files, located in a user-provided directory.

Cookies and HTTP
The connection established between the server and the client uses a Hypertext Transfer Protocol (HTTP). Although this protocol is very complicated at the implementation level, it is fairly easy to understand conceptually. When a user requests a page, an HTTP request is sent to the server, specifying the user’s exact request with some additional attributes. As a user, you are not aware of any data sent to the server as a result of your request. Among its elements, an HTTP request includes a header that

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defines the most important attributes, such as the URL of the requested page. An HTTP request includes all valid cookies as well. When the server replies to the client’s request, it returns an HTTP response which also features a header. This header contains important information about the file being returned, such as its MIME types (discussed later in the book). The general syntax of an HTTP header is as follows:
Field-name: Information

When the server returns an HTTP object to the client, it may also transmit some state information for the client to store as cookies. Since a cookie is basically simple text, the server-side script does not have the ability to abuse the client machine in any way. In addition to its textual value, a cookie contains several attributes, such as the range of URLs for which the cookie is valid. Any future HTTP requests from the client to one of the URLs in the above range will transmit back to the server the current cookie’s value on the client.

Setting an HTTP Cookie
An HTTP cookie is introduced to the client in an HTTP request, usually by a CGI script, using the following syntax:
Set-Cookie: NAME=VALUE; expires=DATE; path=pathName; domain=DOMAIN_NAME; secure

The attributes are as follows:
NAME=VALUE

name is the name of the cookie by which you can reference it later. Notice that the only way to access the cookie is by this name. value is the regular string to be stored as a cookie. It is recommended that the string be encoded using the “%XX” style (equivalent to JavaScript’s escape function’s output). Generally speaking, the name=value is the only required attribute of the Set-Cookie field.
expires=DATE

expires is an optional attribute that specifies the expiration date of the cookie. The cookie will no longer be stored or retrieved beyond that date. The date string is formatted as follows:
Wdy, DD-Mon-YYYY HH:MM:SS GMT

You will see later that this date format is equivalent to the value returned by the toGMTString() date’s method. If expires is not specified, the cookie will expire when the user’s session ends.
path=pathName

path specifies a subset of URLs in a domain for which a cookie is valid. After domain matching, the pathname component of the URL is compared with the path attribute, and, if successful, the cookie is considered valid and is sent along with the URL requests. The path /foo, for example, would match /foobar and /foo/bar/html. The

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path / is the most general one. If the path is not specified, it is assumed to be the same path as the document specified in the cookie’s header.
domain=DOMAIN_NAME

When searching for valid cookies, your browser compares the domain attributes of each cookie to the Internet domain name of the host from which the URL will be retrieved. If there is a tail match, the cookie will go through a full path matching. Tail matching means that the domain attribute is matched against the tail of the fully qualified domain name of the host. A domain attribute of “ac.il”, for example, would tail match “mis.study.ac.il” as well as “mba.haifa.ac.il”. The domain attribute makes sure that only hosts within the specified domain can set a cookie for the domain. There are seven common top-level domains: “com”, “edu”, “net”, “org”, “gov”, “mil”, and “int”, along with several others that have been proposed recently. The default value of domain is the host name of the server that generated the cookie response.
secure

If a cookie is marked secure, it will only be transmitted across a secured communication channel between the client and the host. If secure is not specified, the cookie will be sent over unsecured channels.

Getting an HTTP Cookie
When a script (client-side or server-side) requests a URL from an HTTP server, the browser will match the URL against all cookies, and if any of them matches, a line containing the name and value pairs of all matching cookies will be included in the HTTP request. The format is straightforward:
Cookie: NAME1=VALUE1; NAME2=VALUE2 …

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Notice that the Cookie field in a request header contains only the names and values of all valid cookies. The Set-Cookie field in the response header includes additional attributes such as expiration date. These attributes are not actually part of the cookie, but rather are used to determine if a specific cookie is valid for the purpose of entering the HTTP request header.

Notes and Limitations
The only way to overwrite a cookie is by creating another cookie with the same name and path as an existing one. Creating a cookie with the same name but with a different path than that of an existing one will add an additional cookie. The only way to instantly delete a cookie is by overwriting it with an expired cookie. A cookie may be deleted by the browser before its expiration date but only if the number of cookies exceeds its internal limit.

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When sending cookies to a server, all cookies with more specific path mapping should be sent before cookies with less-specific path mapping. If both are sent, the cookie name1=foo with a path mapping of /, for example, should be sent after the cookie name1=foo2 with a path mapping of /bar. There are several extremely important limitations on the size and number of cookies a client can store at any given time: n The client can hold up to 300 cookies. n A cookie can be up to 4 KB, including its name and values. Cookies that exceed this length are trimmed to fit, so remember to keep within this length. n A maximum of 20 cookies per server or domain are allowed. A client is not expected to exceed these limits. The oldest cookies are deleted in case this rule is violated. Proxy servers should propagate the Set-Cookie header to the client, regardless of whether the response was 304 (“not modified”) or 200 (“OK”). Proxy servers work fine with cookies.

Examples
Here are some sample exchanges from Netscape documentation that illustrate the use of cookies. First Transaction Sequence Example Client requests a document and receives in the response:
Set-Cookie: CUSTOMER=WILE_E_COYOTE; path=/; expires=Wednesday, 09-Nov-99 23:12:40 GMT

When client requests a URL in the path / on this server, it sends:
Cookie: CUSTOMER=WILE_E_COYOTE

Client requests a document and receives in the response:
Set-Cookie: PART_NUMBER=ROCKET_LAUNCHER_0001; path=/

When client requests a URL in the path / on this server, it sends:
Cookie: CUSTOMER=WILE_E_COYOTE; PART_NUMBER=ROCKET_LAUNCHER_0001

Client receives:
Set-Cookie: SHIPPING=FEDEX; path=/foo

When client requests a URL in the path / on this server, it sends:
Cookie: CUSTOMER=WILE_E_COYOTE; PART_NUMBER=ROCKET_LAUNCHER_0001

When client requests a URL in the path /foo on this server, it sends:
Cookie: CUSTOMER=WILE_E_COYOTE; PART_NUMBER=ROCKET_LAUNCHER_0001; SHIPPING=FEDEX

Second Transaction Sequence Example Assume all mappings from above have been cleared.

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Client receives:
Set-Cookie: PART_NUMBER=ROCKET_LAUNCHER_0001; path=/

When client requests a URL in the path / on this server, it sends:
Cookie: PART_NUMBER=ROCKET_LAUNCHER_0001

When client requests a URL in the path /ammo on this server, it sends:
Cookie: PART_NUMBER=RIDING_ROCKET_0023; PART_NUMBER=ROCKET_LAUNCHER_0001

Note that there are two attributes named PART_NUMBER due to the two different paths, / and /ammo.

Cookies and JavaScript
Setting and getting cookies with a server-side application relies on HTTP headers. You cannot set a cookie or retrieve one after the page has loaded. However, a JavaScript script is a client-side application and thus enables you to process cookies at any time, without contacting the server. The cookie property of the document object reflects all cookies that are valid for the web page hosting the script; that is, document.cookie is equivalent to the Cookie field in the HTTP request header. In the same way you set a cookie via the Set-Cookie field in an HTTP response header, you can do so with JavaScript, by assigning a value to document.cookie.

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Cookie Functions
You should have noticed that we neither demonstrated nor emphasized the usage of cookies with JavaScript. The reason for this is that it is difficult and almost useless to explicitly deal with the document.cookie property. Furthermore, there are only three defined actions related to cookies: n Retrieving the value of a cookie according to its name n Setting a cookie to a desired attribute n Deleting a cookie You may find the following function useful, when you want to perform one of the above actions. All functions are self-explanatory and are fully commented:
// Boolean variable specified if alert should be displayed if cookie exceeds 4KB var caution = false // name - name of the cookie // value - value of the cookie // [expires] - expiration date of the cookie (defaults to end of current session)

Bear in mind that this script does not emulate a regular counter that counts the number of visitors to a given site. Because cookies are stored on the client side, they can only be used to count the number of visits by a specific client. At first, an instance of the Date object reflecting the current date is assigned to the variable now. The instance is then handed to the fixDate() function, which is needed to properly format the date values for users who have the Macintosh operating system. The now.setTime(now.getTime() + 365 * 24 * 60 * 60 * 1000) statement sets the cookie expiration date to one year in the future. This expiration date is used later when setting the cookie. The script gets a cookie named counter and retrieves the number of visits from it. If there is no cookie by the specified name, the variable visits is initialized to one; otherwise, the number of visits in the cookie is incremented by one. The script then writes the new number of visits back to the cookie by setCookie(). The last statement of the script informs the user how many visits the user had in the current page. The following script is similar to the preceding one, but it asks the user for his or her name and “remembers” it. Here is the script:
<HTML> <HEAD> <TITLE>Remember user's name</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-// Boolean variable specified if alert should be displayed if cookie exceeds 4KB var caution = false // name - name of the cookie // value - value of the cookie // [expires] - expiration date of the cookie (defaults to end of current session) // [path] - path for which the cookie is valid (defaults to path of calling document) // [domain] - domain for which the cookie is valid (defaults to domain of calling

Example 22-2. A script that remembers the user’s name and displays a greeting each time.

Figure 22-2. The script requests the user’s name...

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Figure 22-3. and then greets the user.

The differences between this script and the previous one are confined to the last five statements of the script. In the current script, the name of the cookie is name and it is the user’s name that is being retrieved from it. Notice that the variable to which you assign the value of getCookie() does not have to have the same name as the cookie, as is done here. If the cookie is not found, the user is prompted for his or her name.

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The script then sets the cookie with the user’s name and prints a personalized welcome message. Examples 22-1 and 22-2 were rather simple, because they only set and read a cookie. Example 22-3 demonstrates using cookies in a slightly different way. It creates a calendar that enables the user to enter reminders for specific days of the month. This reminder calendar stores the data for the entire month, even if the user turns off his or her computer. Since there are up to 31 days in a month, and since you are limited to 20 cookies per domain or server, the script stores all reminders in a single cookie, with special delimiting sequences. Assuming a reminder does not exceed 100 characters, storing 31 values takes up 31 * 100, or 3100 characters, which is less than the 4KB upper limit. Here is the script:
<HTML> <HEAD> <TITLE> JavaScript calendar </TITLE> </HEAD> <BODY> <SCRIPT LANGUAGE="JavaScript"> <!-// // Cookie functions to store and retrieve cookies // // Boolean variable specified if alert should be displayed if cookie exceeds 4KB var caution = false // name - name of the cookie // value - value of the cookie // [expires] - expiration date of the cookie (defaults to end of current session) // [path] - path for which the cookie is valid (defaults to path of calling document) // [domain] - domain for which the cookie is valid (defaults to domain of calling document) // [secure] - Boolean value indicating if the cookie transmission requires a secure transmission // * an argument defaults when it is assigned null as a placeholder // * a null placeholder is not required for trailing omitted arguments function setCookie(name, value, expires, path, domain, secure) { var curCookie = name + "=" + escape(value) + ((expires) ? "; expires=" + expires.toGMTString() : "") + ((path) ? "; path=" + path : "") + ((domain) ? "; domain=" + domain : "") + ((secure) ? "; secure" : "") if (!caution || (name + "=" + escape(value)).length <= 4000) document.cookie = curCookie else if (confirm("Cookie exceeds 4KB and will be cut!")) document.cookie = curCookie } // name - name of the desired cookie // * return string containing value of specified cookie or null if cookie does not exist

This statement differs from its corresponding one in Example 12-3 in that, instead of writing the date as a plain number, it makes each number a link that invokes the getInput() function, using the javascript:getInput() URL. The exact syntax of the URL is as follows:
javascript:getInput(digit, monthName)

The first argument is the digit that serves as a link (an integer from 1 to 28, 29, 30, or 31). The second argument is a string—the full name of the current month reflected by the calendar. Note that when the user places the mouse pointer over a link, a related message is assigned to the status bar. We will only discuss the functions that are responsible for the cookie handling and storage because the functions that create the calendar are discussed in Chapter 12.

The getInput() function, in general, reads the reminder from the cookie, asks the user to modify it or enter it for the first time, and then saves the reminder back to the cookie. First, it gets the reminder text from the cookie, the name of which is composed of the name of the current month and the “Calendar” string. The name selection algorithm prevents mixing cookies between different months or different applications. If the cookie is not found, the function initCookie() is called to create a cookie with empty reminders, one for each day of the month. The script then prompts the user for a reminder, displaying the old one in the form’s input field. Notice that the reminder is displayed in the field of a prompt dialog box by specifying the value returned by getSpecificReminder(), as the second argument for the prompt() method. If the user did not press Cancel as a response to the request form, the new information is saved in the cookie by the setSpecificReminder() function.

The initCookie() function creates a cookie with empty reminders, one for every day of the month. The cookie’s text is a string concatenation of all 31 reminders, delimited by “^” from each other. The function first builds the empty string, fixes the date for Mac computers, computes the time, and finally sets the cookie, saving the empty reminders, date, and time in it.

This function retrieves the specific reminder of the month from the cookie. It first builds the “search key” in prefix. It is built of a “^” followed by the day number of the month and another “^”. After getting the cookie and reading the string into the totalCookie string object, the function searches for the position of the relevant reminder, according to prefix “search key,” yielding the startIndex position. Looking for the next “^” skips over the number of the day itself and leaps onto the string index of the reminder’s first character, startData. The last character of the reminder is found via a search for the next “^” character or by reaching the end of the cookie. Once the startData and endData are known, the substring between these two indices is returned. Note that because the indexing relies on “^” characters, that character should not be provided by the user in a reminder text.

As in setSpecificReminder(), the first section determines the indices of the first and last character of the relevant reminder (startData and endData, respectively). An instance of the Date object is then created and fixed for Mac computers. The

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expiration date is then computed to one month in the future. The last statement of the function sets the cookie. The first parameter is the name of the cookie, while the second one is the reminder string, composed of the substring before the new reminder (all previous days are not modified), the newValue of the current day, and the rest of the old string (all following days are not modified as well). The expiration date is handed to the function as the last argument.

Outliner
As you have seen in this chapter, you can use JavaScript to store data in the form of client-side cookies. The outliner is another example for using cookies. It is an expanding/collapsing structure used to store an index or table of contents. The user can expand or collapse items in the outline. Outliners first became popular in Windows 3.1 and are still used today. An outliner written in JavaScript enables the user to take advantage of the structure for convenient navigation among many web pages and anchors. The main topics of a web site are the topmost items in the outline, while the more detailed items are usually deeply nested. Take a look at the following:

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Figure 22-4. A fully expanded outline.

Figure 22-4 illustrates a fully expanded outline. When the user clicks the downwards triangle to the left of “software,” the icon becomes a triangle facing right. The “Netscape” and “Microsoft” items then disappear, because their parent was collapsed. The basic idea should be clear. You can toggle a single item (only if it is a parent) between two different states (either expanded or collapsed).

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Before we discuss the script itself, it is important that you understand exactly what the script does and how it differs from other JavaScript outliners available. Compared to other variants you might find on the web, this outliner script has the following advantages: n You can use any HTML tag for an item. You can use colored text, hypertext links, and even small images. n The current state of the outline structure is stored in the form of a cookie, so it is safe, even if you leave the page. n You can use as many nested items as you wish. Some outline designs limit you to two levels, which is very restrictive. n The outliner design is not limited to frame documents. Since you can use any HTML for the text of an item, you can target any link to any frame or window you wish. First, study the script (Example 22-4) and try to understand as much as you can:
<HTML> <HEAD> <TITLE>Outliner</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-// Boolean variable specified if alert should be displayed if cookie exceeds 4KB var caution = false // name - name of the cookie // value - value of the cookie // [expires] - expiration date of the cookie (defaults to end of current session) // [path] - path for which the cookie is valid (defaults to path of calling document) // [domain] - domain for which the cookie is valid (defaults to domain of calling document) // [secure] - Boolean value indicating if the cookie transmission requires a secure transmission // * an argument defaults when it is assigned null as a placeholder // * a null placeholder is not required for trailing omitted arguments function setCookie(name, value, expires, path, domain, secure) { var curCookie = name + "=" + escape(value) + ((expires) ? "; expires=" + expires.toGMTString() : "") + ((path) ? "; path=" + path : "") + ((domain) ? "; domain=" + domain : "") + ((secure) ? "; secure" : "") if (!caution || (name + "=" + escape(value)).length <= 4000) document.cookie = curCookie else if (confirm("Cookie exceeds 4KB and will be cut!")) document.cookie = curCookie } // name - name of the desired cookie // * return string containing value of specified cookie or null if cookie does not exist function getCookie(name) {

A quick look at Example 22-4 is enough to conclude that the script uses cookies. Our set of cookie-related functions is embedded in the script. Although not all are invoked, we feel that you should always provide all the functions, regardless of whether you require all of them. Also notice that the HTML document includes two scripts—one in the <HEAD>…</HEAD> portion and one in the <BODY>…</BODY> portion. The first script is responsible for setting up the outline-style table and contents, and for performing the calculations and manipulations. The second script simply interprets the data and prints it to the web page. First let’s take a look at the script in the <HEAD>…</HEAD> portion.

The constructor function item() accepts three arguments and creates three properties with the same names as the function’s parameters: parent, text, and depth. The first parameter (and property), parent, accepts a Boolean value indicating whether or not the item is a parent. An item is considered a parent only if it has children, or nested items. The function’s second argument, text, accepts an HTML-valid string such as <A HREF="http://www.jumbo.com">Jumbo</A> for the “Jumbo” entry in Figure 22-4. You can use virtually any HTML-valid string such as plain text, a link (as in the “Jumbo” example), or even a small image. The last argument, depth, is the depth of the item being created, i.e., it specifies how deep the item is nested. The topmost level item’s depth property is 0, whereas its children’s depth property is 1, and so forth.

This function is an alternative to the built-in Array() constructor of Navigator and MSIE. We decided to write an array constructor on our own in order to make the script compatible with all JavaScript-enabled browsers. We used JavaScript’s support for adding properties to an object, to explicitly create length, which holds the number of elements in the array, or the number of its properties (not counting length itself), to be exact.

This function creates the main array used for the outline structure. Notice that the outline array is declared without the keyword var and thus is global. Try adding var to this statement to find out how important “one small word” can be. The next portion of this function deals with the properties (elements) of the outline array. Each property becomes an instance of the item object. The order of the elements in the array is very important. An item’s children should immediately follow the item, so the order in the outline array is the order in which the entries appear, from top to bottom, in a fully expanded table of contents. The function then calls setStates() and setImages().

The setStates() function adds a state property to all properties of the outline object (array). If no cookie by the name of “outline” is found, the default states are used, i.e., the entire outline structure is collapsed, and only the topmost level is viewable. If the desired cookie is found, the current states are extracted. The cookie is basically a string of “0” and “1” characters, the first representing a false state and the latter representing a true one. The first character of the string is associated with the first element of the array (outline[0].state), and so on. An item’s state determines whether or not that item’s parent is expanded. Therefore, on a two-level outline-style table of contents, if an item has a true state, it is viewable.

This function loops through all elements of the outline object and assigns an image to the pic property of each element whose state is true. Note that this property is an extension of the original instance of the item object. If an element is a parent, there are two possible images—one to represent an expanded item and the other to reflect a collapsed one. Notice that if the Boolean expression outline[i + 1].state is true, outline[i] is expanded. You may recall that an element’s state property is true if its parent is expanded (by definition). Since outline[i] is surely a parent in this case (if (outline[i].parent)…), outline[i + 1] is its child. If outline[i + 1]’s current state is false, the image representing a collapsed item is chosen. If outline[i] is not a parent at all, there is only one option for the image—the one representing a child item. The immediate conclusion from this function is that in order to use the outliner, you must have three images: n exploded.gif n collapsd.gif n child.gif

When the user clicks an image (exploded.gif or collapsd.gif), the item associated with it either collapses or explodes, depending on its current status. The toggle() function accepts an integer reflecting the index of the item whose icon the user clicked, and then toggles the state property of all the item’s direct children. By definition, when an item has no children with a true state, the item is collapsed, and when all children of an item have a true state, the item is expanded. Therefore, toggling the state property of the selected item’s children toggles that item’s current status (exploded or collapsed). After all manipulations, the function invokes the setStorage() function to store the current status in a cookie, overwriting any previous cookie used for the outliner. After the cookie is written, the function reloads the page via the history.go() method, with the argument 0 to indicate that the current page should be loaded. We chose to use go(0) rather than reload() because it works with all JavaScript-compatible browsers, and it is perfect for refreshing a document, which is exactly what the script does.

The setStorage() function creates a string with binary characters (0s and 1s). The first character of the string is associated with the first element of the outline array, and so forth. The character “0” indicates that the value of the item’s state property is false, and a “1” character means that the item’s state property is true. The last statement of this function sets the cookie via the setCookie() function, using “outline” as the name and the accumulative string as the value. No path or expiration date is specified, so the cookie is specific to the creating page and expires at the end of the user’s current session.

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Global Statements
The only global statement in the first script is the one invoking the makeDatabase() function. The second script, on the other hand, consists of global statements only and is responsible for printing the outline-style table of contents. The <PRE> and </PRE> tags are important because they enable us to use regular spaces for indentation. The most important statement in the second script is the loop itself, which iterates through all elements of the global outline array (created by the makeDatabase() function in the first script). Each indentation level consists of three spaces and can be configured to any other integer for customized indentation. The topmost level items are not indented at all (0 * 3 = 0), the second level is indented by three spaces (1 * 3 = 3), the third level by six spaces (2 * 3 = 6), and so on. Note that an item is only printed if the value of its state property is true (by definition, if it is false, its parent is collapsed so you are not supposed to see the item). Each printed item consists of its small image (outline[i].pic), followed by one space and its text (outline[i].text). A new line (<BR>) is appended to each item. When an element whose state property is false is encountered, i is incremented the desired amount of times until its index is that of the next item at its level or a higher one. It passes over all items at lower levels because they do not appear.

Summary
In this chapter you have learned the fundamentals and usage of cookies, one of the most powerful features of JavaScript. It enables the programmer of a web page to store information in one session and retrieve it later in another one. I have provided three important functions by which you can get a cookie, set a cookie, and delete a cookie. All operations, as well as storage, are handled on the client side. Cookies are transparent to the server. The client imposes two limitations on cookies: a maximum of 20 cookies per server or domain, and 4KB each. I have shown several scripts in this chapter: one that uses cookies to remember the number of visits to a web site, one that uses cookies to store the user name, and one that uses cookies to remember a user’s reminder for every day of the month. I also included an advanced collapsing/expanding outline-style table of contents which uses cookies as well.

The attributes are: NAME="imageName" specifies the name of the image object. SRC="Location" specifies the URL of the image to be displayed in the document. LOWSRC="Location" specifies the URL of a low-resolution version of the image to be displayed in the document. When this argument is provided, the smaller image is loaded first, and then replaced with the larger image specified by SRC. Loading a low-resolution version first gives the user the impression of a shorter turnaround time. HEIGHT="Pixels" | "Value"% specifies the height of the image, either in pixels or as a percentage of the window height. If necessary, the image is scaled to fit the space specified by this attribute.

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WIDTH="Pixels" | "Value"% specifies the width of the image, either in pixels or as a percentage of the window width. If necessary, the image is scaled to fit the space specified by this attribute. HSPACE="Pixels" specifies the margin in pixels between the left and right edges of the image and the surrounding text. This attribute applies only to images that use “left” or “right” as the value of the ALIGN attribute. VSPACE="Pixels" specifies the margin in pixels between the top and bottom edges of the image and the surrounding text. This attribute applies only to images that use “top” or “bottom” as the value of the ALIGN attribute. BORDER="Pixels" specifies the width in pixels of the image border. You can suppress the border by setting its value to 0. If, however, it appears within an anchor, users will not see the colored border indicating a hyperlink. ALIGN specifies the alignment of the image in relation to the surrounding text. Images that are aligned as left or right float into the next available space on the left or right side of the page, respectively, while text fills the empty space next to the image. The rest of the ALIGN values specify the alignment of the image with respect to a line of text in which it is placed (no filling). If omitted, "bottom" is used, which means that the bottom of the image is aligned with the line of text. ISMAP specifies the image as a server-side image map. USEMAP="Location#MapName" specifies the image as a client-side image map. This attribute must specify the URL of the file that contains the map definition, followed by a # symbol, and then the name of the map. For example, USEMAP="http://www.HomeWorld.com/maplist.html#areamap". The URL can be omitted if the image map specifications reside in the same document as the reference.

The Image Object
The Image object enables you to create instances that reflect a given image in any supported format (usually gif or jpeg). You should be aware that very early versions of Netscape Navigator and Internet Explorer do not support this object. Users using versions 3.0 and earlier will get an error. By introducing movement and animation, the Image object immensely increased JavaScript’s capabilities. You can take advantage of this object to create an animation, for example, with full control over timing and order of events. You can also create animation-based games such as Tetris, MineSweeper, and so forth. The primary incentive for using the Image object is to accelerate image displaying in the browser window. Instead of waiting for the image to be transmitted from the server to the client when the display is needed, it is loaded and stored in the browser’s cache ahead of time, and displayed immediately upon request. In order to use the Image object, you must create an instance associated with a given image. The general syntax is as follows:

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var imageName = new Image([width, height])

width is the width of the image in pixels, and height is its height. An instance of the Image object can be associated with one image at any given time. In order to associate an instance with an existing image you must assign it a source in the following fashion:
var imageName = new Image([width, height]) imageName.src = "imageLocation"

imageLocation is the full URL of the image. The second statement in the preceding script segment assigns a value to the instance’s src property. The browser will retrieve the image from the server and will keep it in the cache until needed. Note that the width and height attributes are optional.

The images Array
When you create an HTML document, you usually include several images (defined by the <IMG> definition). JavaScript features an array that reflects all images in a document—document.images. Each element of the array reflects an existing image. The first image in a document, for example, is document.images[0]. Obviously, the total number of images in a document is stored in the length property—document. images.length. Using the array within a deferred script ensures that all images have been loaded and the array reflects all images of a document. An alternative way to reference an image is by its name, which is defined by the NAME attribute of the <IMG> HTML tag. See Example 23-5 for further details and explanations. The size and position of an image in a document are set when the document is displayed in the browser window, and cannot be changed. Therefore, when creating an animation, you should generally use images of the same height and width. You can only change the image itself by setting the src and lowsrc properties. (See the listings for the SRC and LOWSRC attributes in the section “Defining Images in HTML.”) Consider the following HTML document:
<HTML> <HEAD> <TITLE>images</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function swapImages(a, b) { var asource = document.images[a].src document.images[a].src = document.images[b].src document.images[b].src = asource } // --> </SCRIPT> </HEAD> <BODY>

When the user clicks the button, the swapImages() function is invoked with two arguments: 0 and 1. The src property of the image whose index is the first argument is assigned to the local variable source. The src property of the second image is assigned to the src property of the first image, and the src property of the second image is assigned the previous src property of the first image, stored locally in asource. The document.images array is read-only—you cannot explicitly assign a value to any of its elements. Nevertheless, you can assign values to properties of an array element, as the preceding example demonstrates with the src property. As mentioned earlier, an animation should consist of images of the same size. Example 23-2 demonstrates a simple animation with seven images of identical dimensions:
<HTML> <HEAD> <TITLE>images</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-var pause = 250 var on = new Array()

The script consists of a few immediate statements as well as deferred code. First, an array named on is created to store the images of the animation. Each element is actually an instance of the Image object. Take a look at the following statement:
timerID = setTimeout("", 0)

The animate() function accepts two arguments. The first specifies the index of the first image in the animation according to the on array. Take a look at the following statement from animate(), which is the most important one:
document.images[imageIndex].src = on[num].src

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The current image that appears in the document at the imageIndex index of the document.images array is replaced by the image in the on array, whose index is equal to the first argument handed to the function. The second statement in this function sets the value of num to 0 if the current value exceeds the index of the last entry in the on array. Otherwise, it increments the value of num, so the following image is displayed during the next execution of the function.

Properties
Instances of the Image object feature many properties, some of which are more useful than others. You can also add more properties by creating prototypes. This section describes each property in depth. border An image’s border appears only when the image is used in a hypertext link and when the value of the BORDER attribute is set to a positive integer. The general reference is as follows:
imageName.border

imageName is either the name of an Image object’s instance or an element in the document.images array. The border property is read-only. The following function displays the image’s border if it is not 0:
function checkBorder(theImage) { if (theImage.border == 0) alert('The image has no border!') else alert('The image's border is ' + theImage.border) }

complete The complete property is a Boolean value that indicates whether the browser has completed its attempt to load an image. The general specification is as follows:
imageName.complete

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imageName is either the name of an Image object’s instance or an element in the document.images array. height The height property specifies the height of an image, either in pixels or as a percentage of the window’s total height. The general syntax is as follows:
imageName.height

imageName is either the name of an Image object’s instance or an element in the document.images array. The height property reflects the HEIGHT attribute of the <IMG> tag. For images created with the Image() constructor, the value of the height property is the actual height, not the displayed height. The height property is read-only.

Example 23-3. A script to display the height, width, and space around an image.

Figure 23-2. An alert box that displays the height, width, and space around an image.

hspace The hspace property specifies the margin in pixels between the left and right edges of an image and the surrounding text. The general syntax is as follows:

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imageName.hspace

imageName is either the name of an Image object’s instance or an element in the document.images array. The hspace property reflects the HSPACE attribute of the <IMG> tag. For images created with the Image() constructor, the value of the hspace property is 0. The hspace property is read-only. The script in Example 23-3 shows how an alert box can display, upon clicking a button, the height, width, and space figures of an image. lowsrc lowsrc is a string specifying the URL of a low-resolution version of an image to be displayed in a document. The general syntax is as follows:
imageName.lowsrc

imageName is either the name of an Image object’s instance or an element in the document.images array. The lowsrc property initially reflects the LOWSRC attribute of the <IMG> tag. The smaller image specified by lowsrc is loaded and then replaced with the larger image specified by the src property. You can change the lowsrc property at any time. The script in Example 23-4 lets the user display one image out of three available ones. There are two images ready for each of the three selections: low resolution and high resolution. When loading the requested selection, the low-resolution image is loaded first and then the high-resolution one:
<HTML> <HEAD> <TITLE>Aircrafts</TITLE> <SCRIPT LANGUAGE="JavaScript"> <!-function displayImage(lowRes, highRes) { document.images[0].lowsrc = lowRes document.images[0].src = highRes } // --> </SCRIPT> </HEAD> <BODY> <FORM NAME="imageForm"> <B>Choose an image:</B> <BR><INPUT TYPE="radio" NAME="imageChoice" VALUE="image1" CHECKED onClick="displayImage('img1l.gif','img1h.gif')"> IMAGE 1 <BR><INPUT TYPE="radio" NAME="imageChoice" VALUE="image2" onClick="displayImage('comp1.gif', ‘comp2.gif')"> IMAGE 2 <BR><INPUT TYPE="radio" NAME="imageChoice" VALUE="image3" onClick="displayImage('img3l.gif', 'img3h.gif')"> IMAGE 3 <BR> <IMG NAME="firstImage" SRC="img1h.gif" LOWSRC="img1l.gif" ALIGN="left" VSPACE="10"><BR> </FORM>

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</BODY> </HTML>

Example 23-4. A script to display one of three images; low-resolution images are loaded first.

name The name property reflects the NAME attribute of the <IMG> HTML definition. The name property is read-only. Similar to forms, you can use an image’s name to reference it. If the first image in a document is defined by the following syntax, for instance, you can reference it as document.myImage as well as document.images[0]:
<IMG SRC="myPicture.gif" NAME="myImage">

src src specifies the URL of an image to be displayed in a document. The general syntax is:
imageName.src

imageName is either the name of an Image object’s instance or an element in the document.images array. The src property is used in almost all of this chapter’s examples. Consider the following statement:
var myImage = new Image()

An instance of the Image object, named myImage, is created with the Image() constructor. When you create an instance of the Image object in this fashion, it is not associated with any image. In order to associate it with an existing image you must assign a value to its src property, in the following manner:
myImage.src = "myPicture.gif"

You can use either a full or relational URL. When you associate an image with an instance in this fashion, the image is cached. Since it is already stored on the client side (where the cache normally is), the user does not have to wait for the image to be received from the server when you decide to display the image. When you adjust the src property of an element from the document.images array (or an image that is viewable on the page), the image immediately changes to the image at the new URL. vspace This property is a string specifying the margin in pixels between the top and bottom edges of an image and the surrounding text. The general syntax is as follows:
imageName.vspace

imageName is either the name of an Image object’s instance or an element in the document.images array. The vspace property reflects the VSPACE attribute of the <IMG> tag. For images created with the Image() constructor, the value of the vspace property is 0. The vspace property is read-only. The script in Example 23-3 shows how an alert box can display, upon clicking a button, the height, width, and space around an image.

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width width is a string specifying the width of an image either in pixels or as a percentage of the window width. The general syntax is as follows:
imageName.width

imageName is either the name of an Image object’s instance or an element in the document.images array. The width property reflects the WIDTH attribute of the <IMG> tag. For images created with the Image() constructor, the value of the width property is the actual, not the displayed, width of the image. The width property is read-only. The script in Example 23-3 shows how an alert box can display, upon clicking a button, the height, width, and space around an image.

Example 23-5. The script from Example 23-4 with the onAbort event handler.

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onError An error event occurs when the loading of an image causes an error. The onError event handler executes JavaScript code when an error event occurs. The onError event handler can be assigned a null value to suppress all error dialogues. When you set imageName.onerror to null, your user won’t see any JavaScript errors caused by the image. An error event occurs only when a JavaScript syntax or run-time error occurs, and not when a Navigator error occurs. If you try to set imageName.src = 'notThere.gif', for instance, and notThere.gif does not exist, the resulting error message is a Navigator error message, and an onError event handler would not intercept that message. In the following <IMG> tag, the onError event handler calls the function badImage if errors occur when the image loads:
<SCRIPT LANGUAGE="JavaScript"> <!-function badImage(theImage) { alert('Error: ' + theImage.name + ' did not load properly.') } // --> </SCRIPT> <IMG NAME="imageBad2" SRC="orca.gif" ALIGN="left" BORDER=2 onError="badImage(this)">

onLoad The onLoad event handler is triggered when an image is displayed. Do not confuse displaying an image with loading one. You can load several images and then, by setting the instance’s src property, you can display them one by one in the same Image object instance. If you change a displayed image this way, the onLoad event handler executes every time an image is displayed, not only when the image is loaded into memory. If you specify an onLoad event handler for an Image object that displays a looping gif animation (multi-image gif), each loop of the animation triggers the onLoad event, and the event handler executes once for each loop. By repeatedly setting the src property of an image’s JavaScript reflection, you can use the onLoad event handler to create a JavaScript animation.

Demonstration 1: Updating Digital Clock
The following JavaScript script displays a digital clock on your page that is updated every minute. The clock includes two digits for the hour, a delimiting colon, two digits for the minute, and am/pm subscript:
<HTML> <HEAD> <TITLE>

Global Statements
The script starts by creating the digit array, holding 14 instances of the Image object, for the digits 0 through 9, am, pm, colon, and blank symbols. All images are 21 pixels high and 16 pixels wide, except for the colon and the blank images which are thinner (9 pixels):
// create array of all digit images var digit = new Array() digit[0] = new Image(16, 21) digit[1] = new Image(16, 21)

Since the artistic representation of each symbol is given in a gif format, we assign a gif filename to the src property of each element of the digit array. The gif files are located in the same directory as the URL, and the naming algorithm is based on concatenating the “dg” substring to the characters represented by the image (0-9, am, pm). The colon symbol is denoted by a “c” character, and the blank by a “b.”
// assign sources to digit image objects (0 - 9) for (var i = 0; i < 10; ++i) { digit[i].src = getPath(location.href) + "dg" + i + ".gif" } // assign sources to other image objects digit[10].src = getPath(location.href) + "dgam.gif" digit[11].src = getPath(location.href) + "dgpm.gif" digit[12].src = getPath(location.href) + "dgc.gif" digit[13].src = getPath(location.href) + "dgb.gif"

Then, we find the current time and store it in six variables, four digits for the hour and minute, one for the ampm value, and one Boolean variable for the blinking colon. After initializing the time variables, the script turns off the colon, which is then ready to be turned on next time:
// set initial time values to current time var hour1 = getHour(0) var hour2 = getHour(1) var minute1 = getMinute(0) var minute2 = getMinute(1) var ampm = getAmpm() var colon = false

We need to probe and remember the number of images already displayed in the document:
// get array substring of first clock image in document.images array var start = document.images.length // number of images in document

When the page is loaded, the script displays the clock. Since JavaScript does not support image creation, all images are constructed via HTML:
// print initial clock var openImage = "<IMG SRC=\"" + getPath(location.href) + "dg"

The timerID variable, which holds the elapsed time before the next clock updating, is initialized to null before the clock starts running. For the same reason, the timerRunning variable is set to false. The function update() starts the infinite loop of running the clock:
var timerID = null var timerRunning = false update()

This function retrieves the current value of each digit (and symbol) in the clock and updates only the necessary images, i.e., only those digits that have been changed since the previous iteration. The blinking colon effect is accomplished by simply reversing the value of colon. Notice that the index of the images array is an offset from the last image of the document, not counting the clock’s images. The variable timerID is now modified to recursively execute the function setClock(), after 1,000 milliseconds.

The getHour() function has been mentioned a lot before. It finds the digit in place position of a two-digit hour representation. Notice the computation to convert a

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24-hour military time notation to 12-hour notation. Pay attention, also, to the concatenation of a null string to force the conversion to a string, needed for the charAt() method. The returned value is converted back to integer format.

The getAmpm() function returns 11 if the current time is P.M., and 10 if it is A.M. Notice that, since a return statement immediately terminates a function, the else keyword is not needed here and is commented out.

The script’s last function, getPath(), extracts the full URL of the document’s directory (or folder). It simply finds the last slash in the URL and returns the substring, starting at the beginning of the URL and ending at its last slash.

Demonstration 2: LED Sign
You have probably seen LED signs in airports and bulletin boards. This LED sign displays a given number of messages, one after the other, and returns to the first one